### 87 articles on Tuesday, September 17

arXiv:1909.06358v1 [pdf, other]
Optical, J, and K light curves of XTE J1118+480 = KV UMa: the mass of the black hole and the spectrum of the non-stellar component
Comments: 24 pages, 16 figures, accepted by MNRAS

Optical, J, and K photometric observations of the KV UMa black hole X-ray nova in its quiescent state obtained in 2017-2018 are presented. A significant flickering within light curves was not detected, although the average brightness of the system faded by $\approx 0.1^m$ during 350 days. Changes in the average brightness were not accompanied with the increase or the decrease of the flickering. From the modelling of five light curves the inclination of the KV UMa orbit and the black hole mass were obtained: $i=74^{\circ}\pm 4^{\circ}$, $M_{BH}=(7.06\div 7.24)M_{\odot}$ dependently on the used mass ratio. The non-stellar component of the spectrum in the range $\lambda=6400\div 22000$\r{A} can be fitted by a power law $F_{\lambda}\sim \lambda^{\alpha}$, $\alpha\approx -1.8$. The accretion disk orientation angle changed from one epoch to another. The model with spots on the star was inadequate. Evolutionary calculations using the "Scenario Machine" code were performed for low mass X-ray binaries, a recently discovered anomalously rapid decrease of the orbital period was taken into account. We showed that the observed decrease can be consistent with the magnetic stellar wind of the optical companion which magnetic field was increased during the common envelope stage. Several constraints on evolutionary scenario parameters were done.

arXiv:1909.06361v1 [pdf, other]
The birth environment of the solar system constrained by the relative abundances of the solar radionuclides
Comments: 9 pages, 2 figures, in press, IAU, Cambridge University Press. arXiv admin note: substantial text overlap with arXiv:1605.07096

The relative abundances of the radionuclides in the solar system at the time of its birth are crucial arbiters for competing hypotheses regarding the birth environment of the Sun. The presence of short-lived radionuclides, as evidenced by their decay products in meteorites, has been used to suggest that particular, sometimes exotic, stellar sources were proximal to the Sun's birth environment. The recent confirmation of neutron star - neutron star (NS-NS) mergers and associated kilonovae as potentially dominant sources of r-process nuclides can be tested in the case of the solar birth environment using the relative abundances of the longer-lived nuclides. Critical analysis of the 15 radionuclides and their stable partners for which abundances and production ratios are well known suggests that the Sun formed in a typical massive star-forming region (SFR). The apparent overabundances of short-lived radionuclides (e.g.\, $^{26} {\rm Al}$, $^{41}{\rm Ca}$, $^{36}{\rm Cl}$) in the early solar system appears to be an artifact of a heretofore under-appreciation for the important influences of enrichment by Wolf-Rayet winds in SFRs. The long-lived nuclides (e.g.\, $^{238}{\rm U}$, $^{244}{\rm Pu}$, $^{247}{\rm Cr}$, $^{129}{\rm I}$) are consistent with an average time interval between production events of $10^8$ years, seemingly too short to be the products of NS-NS mergers alone. The relative abundances of all of these nuclides can be explained by their mean decay lifetimes and an average residence time in the ISM of $\sim200$ Myr. This residence time evidenced by the radionuclides is consistent with the average lifetime of dust in the ISM and the timescale for converting molecular cloud mass to stars.

arXiv:1909.06367v1 [pdf, other]
Using Deep Learning to Localize Gravitational Wave Sources

In this paper, we report on the construction of a deep Artificial Neural Network (ANN) to localize simulated gravitational wave signals in the sky with high accuracy. We have modelled the sky as a sphere and have considered cases where the sphere is divided into 18, 50, 128, 1024, 2048 and 4096 sectors. The sky direction of the gravitational wave source is estimated by classifying the signal into one of these sectors based on it's right ascension and declination values for each of these cases. In order to do this, we have injected simulated binary black hole gravitational wave signals of component masses sampled uniformly between 30-80 solar mass into Gaussian noise and used the whitened strain values to obtain the input features for training our ANN. We input features such as the delays in arrival times, phase differences and amplitude ratios at each of the three detectors Hanford, Livingston and Virgo, from the raw time-domain strain values as well as from analytical versions of these signals, obtained through Hilbert transformation. We show that our model is able to classify gravitational wave samples, not used in the training process, into their correct sectors with very high accuracy (>90%) for coarse angular resolution using 18, 50 and 128 sectors. We also test our localization on a test sample with injection parameters of the first BBH merger event detected by LIGO, GW150914, for 1024, 2048 and 4096 sectors and show that for Gaussian noise and advanced LIGO power spectral density, our model is able to constrain the 90% contour area to 312 sq. degrees at SNR of 24. We also report that the time taken by our model to localize one GW signal is around 0.018 secs on 14 Intel Xeon CPU cores.

arXiv:1909.06373v1 [pdf, other]
Machine-learning interpolation of population-synthesis simulations to interpret gravitational-wave observations: a case study

We report on advances to interpret current and future gravitational-wave events in light of astrophysical simulations. A machine-learning emulator is trained on numerical population-synthesis predictions and inserted into a Bayesian hierarchical framework. In this case study, a modest but state-of-the-art suite of simulations of isolated binary stars is interpolated across two event parameters and one population parameter. The validation process of our pipelines highlights how omitting some of the event parameters might cause errors in estimating selection effects, which propagates as systematics to the final population inference. Using LIGO/Virgo data from O1 and O2 we infer that black holes in binaries are most likely to receive natal kicks with one-dimensional velocity dispersion $\sigma$ = 105+44 km/s. Our results showcase potential applications of machine-learning tools in conjunction with population-synthesis simulations and gravitational-wave data.

arXiv:1909.06374v1 [pdf, other]
A forward modelling approach to AGN variability -- Method description and early applications
Comments: Accepter for publication in ApJ. Main figure Fig. 14. Simulations code available at https://github.com/nevencaplar/AGN-Variability-Simulations

We present a numerical framework for the variability of active galactic nuclei (AGN), which links the variability of AGN over a broad range of timescales and luminosities to the observed properties of the AGN population as a whole, and particularly the Eddington ratio distribution function (ERDF). We have implemented our framework on GPU architecture, relying on previously published time series generating algorithms. After extensive tests that characterise several intrinsic and numerical aspects of the simulations, we describe some applications used for current and future time domain surveys and for the study of extremely variable sources (e.g., "changing look" or flaring AGN). Specifically, we define a simulation setup which reproduces the AGN variability observed in the PTF/iPTF survey, and use it to forward model longer light curves of the kind that may be observed within the LSST main survey. Thanks to our effcient implementations, these simulations are able to cover for example over 1 Myr with a roughly weekly cadence. We envision that this framework will become highly valuable to prepare for, and best exploit, data from upcoming time domain surveys, such as for example LSST.

arXiv:1909.06375v1 [pdf, other]
Lifetime of short-period binaries measured from their Galactic kinematics
Comments: Submitted to MNRAS. Figure 5 is the key result

As a significant fraction of stars are in multiple systems, binaries play a crucial role in stellar evolution. Among short-period ($<$1 day) binary characteristics, age remains one of the most difficult to measure. In this paper, we constrain the lifetime of short-period binaries through their kinematics. With the kinematic information from Gaia Data Release 2 and light curves from {\it Wide-field Infrared Survey Explorer} (WISE), we investigate the eclipsing binary fraction as a function of kinematics for a volume-limited main-sequence sample. We find that the eclipsing binary fraction peaks at a tangential velocity of $10^{1.3-1.6}$ km s$^{-1}$, and decreases towards both low and high velocity end. This implies that thick disk and halo stars have eclipsing binary fraction $\gtrsim 10$ times smaller than the thin-disk stars. Using Galactic models, we show that our results are inconsistent with any known dependence of binary fraction on metallicity. Instead, our best-fit models suggest that the formation of these short-period binaries is delayed by $0.6$-3 Gyr, and the disappearing time is less than the age of the thick disk. The delayed formation time of $\gtrsim0.6$ Gyr is too long for any pre-main sequence interaction alone and is more consistent with the three-body interaction through the Kozai-Lidov mechanism and magnetic winds. Because the main-sequence lifetime of our sample is longer than 14 Gyr, if the disappearance of short-period binaries in the old population is due to their finite lifetime, our results imply that most ($\gtrsim90$ %) short-period binaries in our sample are destroyed during their main-sequence stage.

arXiv:1909.06376v1 [pdf, other]
Resolved Lyman-α properties of a luminous Lyman-break galaxy in a large ionised bubble at z = 6.53
Comments: 12 pages, 10 figures. Submitted to MNRAS

The observed properties of the Lyman-$\alpha$ (Ly$\alpha$) emission line are a powerful probe of neutral gas in and around galaxies. We present spatially resolved Ly$\alpha$ spectroscopy with VLT/MUSE targeting VR7, a UV-luminous galaxy at $z=6.532$ with moderate Ly$\alpha$ equivalent width (EW$_0=38$ A). These data are combined with deep resolved [CII]$_{\rm 158 \mu m}$ spectroscopy obtained with ALMA and UV imaging from HST. Ly$\alpha$ emission is clearly detected with S/N $\approx40$ and FWHM of 370 km s$^{-1}$. We also detect UV continuum with MUSE. Ly$\alpha$ and [CII] are similarly extended beyond the UV, with effective radius r$_{\rm eff} = 2.1\pm0.2$ kpc for a single component or r$_{\rm eff, Ly\alpha, halo} = 3.45^{+1.08}_{-0.87}$ kpc when measured jointly with the UV continuum. The Ly$\alpha$ profile is broader and redshifted with respect to the [CII] line (by 220 km s$^{-1}$), but there are spatial variations that are qualitatively similar in both lines and coincide with resolved UV components. This suggests that the emission originates from two components, while spatially varying HI column densities are also present. We place VR7 in the context of other galaxies at similar and lower redshift. The Ly$\alpha$ halo scale length is similar at different redshifts and velocity shifts with respect to the systemic are typically smaller. Overall, we find little indications of a more neutral vicinity at higher redshift. This means that the local ($\sim 10$ kpc) neutral gas conditions that determine the observed Ly$\alpha$ properties in VR7 resemble the conditions in post-re-ionisation galaxies.

arXiv:1909.06378v1 [pdf, other]
Asteroseismic constraints on the cosmic-time variation of the gravitational constant from an ancient main-sequence star
Comments: 5 pages, 3 figures, accepted for publication in The Astrophysical Journal Letters

We investigate the variation of the gravitational constant $G$ over the history of the Universe by modeling the effects on the evolution and asteroseismology of the low-mass star KIC 7970740, which is one of the oldest (~11 Gyr) and best-observed solar-like oscillators in the Galaxy. From these data we find $\dot{G}/G = (2.1 \pm 2.9) \times 10^{-12}~\text{yr}^{-1}$, that is, no evidence for any variation in $G$. We also find a Bayesian asteroseismic estimate of the age of the Universe as well as astrophysical S-factors for five nuclear reactions obtained through a 12-dimensional stellar evolution Markov chain Monte Carlo simulation.

arXiv:1909.06379v1 [pdf, other]
Neural physical engines for inferring the halo mass distribution function

An ambitious goal in cosmology is to forward-model the observed distribution of galaxies in the nearby Universe today from the initial conditions of large-scale structures. For practical reasons, the spatial resolution at which this can be done is necessarily limited. Consequently, one needs a mapping between the density of dark matter averaged over ~Mpc scales, and the distribution of dark matter halos (used as a proxy for galaxies) in the same region. Here we demonstrate a method for determining the halo mass distribution function by learning the tracer bias between density fields and halo catalogues using a neural bias model. The method is based on the Bayesian analysis of simple, physically motivated, neural network-like architectures, which we denote as neural physical engines, and neural density estimation. As a result, we are able to sample the initial phases of the dark matter density field whilst inferring the parameters describing the halo mass distribution function, providing a fully Bayesian interpretation of both the initial dark matter density distribution and the neural bias model. We successfully run an upgraded BORG inference using our new likelihood and neural bias model with halo catalogues derived from full N-body simulations. We notice orders of magnitude improvement in modelling compared to classical biasing techniques.

arXiv:1909.06381v1 [pdf, other]
Dynamical Friction in a Fuzzy Dark Matter Universe

We present an in-depth exploration of the phenomenon of dynamical friction in a universe where the dark matter is composed entirely of so-called Fuzzy Dark Matter (FDM), ultralight bosons of mass $m\sim\mathcal{O}(10^{-22})\,$eV. We review the classical treatment of dynamical friction before presenting analytic results in the case of FDM for point masses, extended mass distributions, and FDM backgrounds with finite velocity dispersion. We then test these results against a large suite of fully non-linear simulations that allow us to assess the regime of applicability of the analytic results. We apply these results to a variety of astrophysical problems of interest, including infalling satellites in a galactic dark matter background, and determine that \emph{(1)}~for FDM masses $m\gtrsim 10^{-21}\, {\rm eV}\, c^{-2}$, the timing problem of the Fornax dwarf spheroidal's globular clusters is no longer solved and \emph{(2)}~the effects of FDM on the process of dynamical friction for satellites of total mass $M$ and relative velocity $v_{\rm rel}$ should require detailed numerical simulations for $\left(M/10^9~M_{\odot}\right) \left(m/10^{-22}~{\rm eV}\right)\left(100~{\rm km}~{\rm s}^{-1}/v_{\rm rel}\right) \sim 1$, parameters which would lie outside the validated range of applicability of any currently developed analytic theory, due to transient wave structures in the time-dependent regime.

arXiv:1909.06382v1 [pdf, other]
Does black-hole growth depend fundamentally on host-galaxy compactness?
Comments: 21 pages, 20 figures. Accepted for publication in MNRAS

Possible connections between central black-hole (BH) growth and host-galaxy compactness have been found observationally, which may provide insight into BH-galaxy coevolution: compact galaxies might have large amounts of gas in their centers due to their high mass-to-size ratios, and simulations predict that high central gas density can boost BH accretion. However, it is not yet clear if BH growth is fundamentally related to the compactness of the host galaxy, due to observational degeneracies between compactness, stellar mass ($M_\bigstar$), and star formation rate (SFR). To break these degeneracies, we carry out systematic partial-correlation studies to investigate the dependence of sample-averaged BH accretion rate ($\rm \overline{BHAR}$) on the compactness of host galaxies, represented by the surface-mass density, $\Sigma_\rm e$, or the projected central surface-mass density within 1 kpc, $\Sigma_1$. We utilize 8842 galaxies with H < 24.5 in the five CANDELS fields at z = 0.5-3. We find that $\rm \overline{BHAR}$ does not significantly depend on compactness when controlling for SFR or $M_\bigstar$ among bulge-dominated galaxies and galaxies that are not dominated by bulges, respectively. However, when testing is confined to star-forming galaxies at z = 0.5-1.5, we find that the $\rm \overline{BHAR}$-$\Sigma_1$ relation is not simply a secondary manifestation of a primary $\rm \overline{BHAR}$-$M_\bigstar$ relation, which may indicate a link between BH growth and the gas density within the central 1 kpc of galaxies.

arXiv:1909.06383v1 [pdf, other]
Intra-cluster GC-LMXB in the Fornax galaxy cluster

The formation of Low mass X-ray binaries (LMXB) is favored within dense stellar systems such as Globular Clusters (GCs). The connection between LMXB and Globular Clusters has been extensively studied in the literature, but these studies have always been restricted to the innermost regions of galaxies. We present a study of LMXB in GCs within the central 1.5 deg^2 of the Fornax cluster with the aim of confirming the existence of a population of LMXB in intra-cluster GCs and understand if their properties are related to the host GCs, to the environment or/and to different formation channels.

arXiv:1909.06385v1 [pdf, other]
Revising natal kick prescriptions in population synthesis simulations
Comments: 12 pages, 5 figures, 2 table, submitted to ApJ. Comments welcome

Natal kicks are matter of debate and significantly affect the merger rate density of compact objects. Here, we present a new simple formalism for natal kicks of neutron stars (NSs) and black holes (BHs). We describe the magnitude of the kick as $v_{\rm kick}\propto{}f_{\rm H05}\,{}\,{}m_{\rm ej}\,{}\,{}m_{\rm rem}^{-1}$, where $f_{\rm H05}$ is a normalization factor, drawn from a Maxwellian distribution with one-dimensional root-mean-square velocity $\sigma{}=265$~km~s$^{-1}$, $m_{\rm ej}$ is the mass of the supernova (SN) ejecta and $m_{\rm rem}$ is the mass of the compact object. This formalism matches the proper motions of young Galactic pulsars and can naturally account for the differences between core-collapse SNe of single stars, electron-capture SNe and ultra-stripped SNe occurring in interacting binaries. Finally, we use our new kick formalism to estimate the local merger rate density of binary NSs ($R_{\rm BNS}$), BH--NS binaries ($R_{\rm BHNS}$) and binary BHs ($R_{\rm BBH}$), based on the cosmic star formation rate density and metallicity evolution. In our fiducial model, we find $R_{\rm BNS}\sim{}600$~Gpc$^{-3}$~yr$^{-1}$, $R_{\rm BHNS}\sim{}10$~Gpc$^{-3}$~yr$^{-1}$ and $R_{\rm BBH}\sim{}50$~Gpc$^{-3}$~yr$^{-1}$, fairly consistent with the numbers inferred from the LIGO-Virgo collaboration.

arXiv:1909.06387v1 [pdf, other]
Hidden Planets: Implications from 'Oumuamua and DSHARP
Comments: 8 pages, 5 figures, accepted to ApJL

The discovery of 'Oumuamua (1I/2017 U1), the first interstellar interloper, suggests an abundance of free-floating small bodies whose ejection into galactic space cannot be explained by the current population of confirmed exoplanets. Shortly after 'Oumuamua's discovery, observational results from the DSHARP survey illustrated the near-ubiquity of ring/gap substructures within protoplanetary disks, strongly suggesting the existence of a vast population of as-yet undetected wide-separation planets that are capable of efficiently ejecting debris from their environments. These planets have $a \gtrsim 5$ au and masses of order Neptune's or larger, and they may accompany $\sim$50% of newly formed stars (Zhang et al. 2018). We combine the DSHARP results with statistical constraints from current time-domain surveys to quantify the population of detectable icy planetesimals ejected by disk-embedded giant planets through gravity assists. Assessment of the expected statistical distribution of interstellar objects is critical to accurately plan for and interpret future detections. We show that the number density of interstellar objects implied by 'Oumuamua is consistent with 'Oumuamua itself having originated as an icy planetesimal ejected from a DSHARP-type system via gravity assists, with the caveat that 'Oumuamua's lack of observed outgassing remains in strong tension with a cometary origin. Under this interpretation, 'Oumuamua's detection points towards a large number of long-period giant planets in extrasolar systems, supporting the hypothesis that the observed gaps in protoplanetary disks are carved by planets. In the case that 'Oumuamua is an ejected cometary planetesimal, we conclude that LSST should detect up to a few interstellar objects per year of 'Oumuamua's size or larger and over 100 yr$^{-1}$ for objects with $r > 1\,{\rm m}$.

arXiv:1909.06388v1 [pdf, other]
Interpreting cosmological tensions from the effective field theory of torsional gravity

Cosmological tensions can arise within $\Lambda$CDM scenario amongst different observational windows, such as measurements of the $H_0$ and $\sigma_8$ parameters. These tensions, if finally confirmed by measurements, may indicate new physics beyond the standard paradigm. In this Letter, we report how to alleviate both the $H_0$ and $\sigma_8$ tensions simultaneously within torsional gravity from the perspective of effective field theory (EFT). We apply the EFT approach, which allows to investigate the evolution equations at the background and perturbation levels in a systematic way, and examine the conditions followed by the coefficients of various possibly involved operators such that cosmological tensions can be relaxed. Following these observations we construct concrete models of Lagrangians of torsional gravity. Specifically, we consider the parametrization $f(T)=-T -2\Lambda/M_P^2 +\alpha T^\beta$, where two out of the three parameters are independent (in which an additional term of the form $cT^{1/2}$ can be added). This model can efficiently fit observations solving all statistical tensions. To our knowledge, this is the first time where a modified gravity theory can alleviate both $H_0$ and $\sigma_8$ tensions simultaneously, hence offering an additional argument in favor of gravitational modification.

arXiv:1909.06390v1 [pdf, other]
Espresso Acceleration of Ultra-High-Energy Cosmic Rays up to the Hillas Limit in Relativistic MHD Jets
Comments: 36th International Cosmic Ray Conference (ICRC2019), held July 24th-August 1st, 2019 in Madison, WI, U.S.A. Online at https://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=358, id.348

Espresso is a novel acceleration model for Ultra-High-Energy Cosmic Rays (UHECRs), where lower-energy CRs produced in supernova remnants experience a one-shot reacceleration in the relativistic jets of powerful Active Galactic Nuclei (AGNs) to reach energies up to $10^{20}$ eV. To test the espresso framework, we follow UHECR acceleration bottom-up from injection to the highest energies by propagating 100,000 particles in realistic 3D magneto-hydrodynamic (MHD) simulations of ultra-relativistic jets. We find that simulations agree well with analytical expectations in terms of trajectories of individual particles. We also quantify that $\sim 10\%$ of CR seeds gain a factor of $\sim\Gamma^2$ in energy, where $\Gamma$ is the jet's effective Lorentz factor; moreover, about $0.1\%$ of the particles undergo two or more shots to achieve gains in excess of $\Gamma^2$. Particles are generally accelerated up to the jet's Hillas limit, indicating that the espresso mechanism should boost galactic CRs to UHECRs in typical AGN jets. Finally, we discuss how espresso acceleration in AGN jets is consistent with UHECR spectra and chemical composition, and also with the UHECR arrival directions measured by Auger and Telescope Array.

arXiv:1909.06391v1 [pdf, other]
Finding the brightest cosmic beacons in the Southern Hemisphere

The study of absorptions along the lines of sight to bright high-$z$ QSOs is an invaluable cosmological tool that provides a wealth of information on the inter-/circum-galactic medium, Dark Matter, primordial elements, reionization, fundamental constants, and General Relativity. Unfortunately, the number of bright ($i \lesssim$ 18) QSOs at $z \gtrsim 2$ in the Southern hemisphere is much lower than in the North, due to the lack of wide multi-wavelength surveys at declination $\delta <$ 0$^\circ$, hampering the effectiveness of observations from southern observatories. In this work we present a new method based on Canonical Correlation Analysis to identify such objects, taking advantage of a number of available databases: Skymapper, Gaia DR2, WISE, 2MASS. Our QSO candidate sample lists 1476 sources with $i < 18$ over 12,400 square degrees in the southern hemisphere. With a preliminary campaign we observed spectroscopically 70 of them, confirming 56 new bright QSOs at $z > 2.5$, corresponding to a success rate of our method of $\sim$ 80\%. Furthermore, we estimate a completeness of $\sim$ 90\% of our sample at completion of our observation campaign. The new QSOs confirmed by this first and the forthcoming campaigns will be the targets of subsequent studies using higher resolution spectrographs, like ESPRESSO, UVES, and (in the long term) ELT/HIRES.

arXiv:1909.06392v1 [pdf, other]
Searching for Black Hole Candidates by LAMOST and ASAS-SN
Comments: 12 pages, 3 figures, accepted for publication in Astronomical Journal

Most dynamically confirmed stellar-mass black holes and the candidates were originally selected from X-ray outbursts. In the present work, we search for black hole candidates in the LAMOST survey by using the spectra along with photometry from the ASAS-SN survey, where the orbital period of the binary may be revealed by the periodic light curve, such as the ellipsoidal modulation type. Our sample consists of 9 binaries, where each source contains a giant star with large radial velocity variation ($\Delta V_{\rm R} > 70~{\rm km~s^{-1}}$) and periods known from light curves. We focus on the 9 sources with long periods ($T_{\rm ph} > 5$ days) and evaluate the mass $M_2$ of the optically invisible companion. Since the observed $\Delta V_{\rm R}$ from only a few repeating spectroscopic observations is a lower limit of the real amplitude, the real mass $M_2$ can be significantly higher than the current evaluation. It is likely an efficient method to place constraints on $M_2$ by combining $\Delta V_{\rm R}$ from LAMOST and $T_{\rm ph}$ from ASAS-SN, particularly by the ongoing LAMOST Medium Resolution Survey.

arXiv:1909.06393v1 [pdf, other]
Two years of non-thermal emission from the binary neutron star merger GW170817: rapid fading of the jet afterglow and first constraints on the kilonova fastest ejecta
Comments: Submitted to ApJ, 13 pages, 6 figures

We present Chandra and VLA observations of GW170817 at ~521-743 days post merger, and a homogeneous analysis of the entire Chandra data set. We find that the late-time non-thermal emission follows the expected evolution from an off-axis relativistic jet, with a steep temporal decay $F_{\nu}\propto t^{-1.95\pm0.15}$ and a simple power-law spectrum $F_{\nu}\propto \nu^{-0.575\pm0.007}$. We present a new method to constrain the merger environment density based on diffuse X-ray emission from hot plasma in the host galaxy and we find $n\le 9.6 \times 10^{-3}\,\rm{cm^{-3}}$. This measurement is independent from inferences based on the jet afterglow modeling and allows us to partially solve for model degeneracies. The updated best-fitting model parameters with this density constraint are a fireball kinetic energy $E_0 = 1.5_{-1.1}^{+3.6}\times 10^{49}\,\rm{erg}$ ($E_{iso}= 2.1_{-1.5}^{+6.4}\times10^{52}\, \rm{erg}$), jet opening angle $\theta_{0}= 5.9^{+1.0}_{-0.7}\,\rm{deg}$ with characteristic Lorentz factor $\Gamma_j = 163_{-43}^{+23}$, expanding in a low-density medium with $n_0 = 2.5_{-1.9}^{+4.1} \times 10^{-3}\, \rm{cm^{-3}}$ and viewed $\theta_{obs} = 30.4^{+4.0}_{-3.4}\, \rm{deg}$ off-axis. The synchrotron emission originates from a power-law distribution of electrons with $p=2.15^{+0.01}_{-0.02}$. The shock microphysics parameters are %loosely constrained to $\epsilon_{\rm{e}} = 0.18_{-0.13}^{+0.30}$ and $\epsilon_{\rm{B}}=2.3_{-2.2}^{+16.0} \times 10^{-3}$. We investigate the presence of X-ray flares and find no statistically significant evidence of $\le2.5\sigma$ of temporal variability at any time. Finally, we use our observations to constrain the properties of synchrotron emission from the deceleration of the fastest kilonova ejecta with energy $E_k^{KN}\propto (\Gamma\beta)^{-\alpha}$ into the environment, finding that shallow stratification indexes $\alpha\le6$ are disfavored.

arXiv:1909.06396v1 [pdf, other]
Systematic-free inference of the cosmic matter density field from SDSS3-BOSS data
Comments: 19 pages, 15 figures, submitted to MNRAS

We perform an analysis of the three-dimensional cosmic matter density field traced by galaxies of the SDSS-III/BOSS galaxy sample. The systematic-free nature of this analysis is confirmed by two elements: the successful cross-correlation with the gravitational lensing observations derived from Planck 2018 data and the absence of bias at scales $k \simeq 10^{-3}-10^{-2}h$ Mpc$^{-1}$ in the a posteriori power spectrum of recovered initial conditions. Our analysis builds upon our algorithm for Bayesian Origin Reconstruction from Galaxies (BORG) and uses a physical model of cosmic structure formation to infer physically meaningful cosmic structures and their corresponding dynamics from deep galaxy observations. Our approach accounts for redshift-space distortions and light-cone effects inherent to deep observations. We also apply detailed corrections to account for known and unknown foreground contaminations, selection effects and galaxy biases. We obtain maps of residual, so far unexplained, systematic effects in the spectroscopic data of SDSS-III/BOSS. Our results show that unbiased and physically plausible models of the cosmic large scale structure can be obtained from present and next-generation galaxy surveys.

arXiv:1909.06403v1 [pdf, other]
Supernova 2014C: ongoing interaction with extended circumstellar material with silicate dust

Supernova (SN) 2014C is a unique explosion where a seemingly typical hydrogen-poor stripped envelope SN started to interact with a dense, hydrogen-rich circumstellar medium (CSM) a few months after the explosion. The delayed interaction suggests a detached CSM shell, unlike in a typical SN IIn where the CSM is much closer and the interaction commences earlier post-explosion; indicating a different mass loss history. We present near- to mid-infrared observations of SN 2014C from 1-5 years after the explosion, including uncommon 9.7 $\mu$m imaging with COMICS on the Subaru telescope. Spectroscopy shows that the interaction is still ongoing, with the intermediate-width He I 1.083 $\mu$m emission present out to our latest epoch 1639 days post-explosion. The last Spitzer/IRAC photometry at 1920 days post-explosion further confirms ongoing CSM interaction. The 1-10 $\mu$m spectral energy distributions (SEDs) can be explained by a dust model with a mixture of 69% carbonaceous and 31% silicate dust, pointing to a chemically inhomogeneous CSM. The inference of silicate dust is the first among interacting SNe. An SED model with purely carbonaceous CSM dust is possible, but would require more than 0.22 $M_{\odot}$ of dust, which is an order of magnitude larger than what observed in any other SNe, measured in the same way, at this epoch. The light curve beyond 500 days is well fit by an interaction model with a wind-driven CSM and a mass loss rate of $\sim 10^{-3} \, M_{\odot}\,\rm yr^{-1}$, which presents an additional CSM density component exterior to the constant density shell reported previously in the literature. SN 2014C could originate in a binary system, similar to RY Scuti, which would explain the observed chemical and density profile inhomogeneity in the CSM.

arXiv:1909.06410v1 [pdf, other]
On Deflection of Solar Coronal Mass Ejections by the Ambient Coronal Magnetic Field Configuration

Solar Coronal Mass Ejections (CMEs) are sometimes deflected during their propagation. This deflection may be the consequence of interaction between a CME and a coronal hole or the solar wind. We analyze 44 halo-CMEs whose deflection angle exceeds 90 degrees. The coronal magnetic field configuration is computed from daily synoptic maps of magnetic field from SOHO/MDI and SDO/HMI using a Potential Field Source Surface (PFSS) model. By comparing the ambient magnetic field configuration and the measured position angles (MPA) of the CMEs, we conclude that the deflection of 80% of the CMEs (35 of 44) are consistent with the ambient magnetic field configuration, agreeing with previous studies. Of these 35, 71% are deflected toward the heliospheric current sheet (HCS), and 29 degrees toward a pseudo-streamer (PS), the boundary between the same-polarity magnetic field regions. This implies that the ambient coronal magnetic field configuration plays an important and major role in the deflection of CMEs, and that the HCS configuration is more important than PS. If we exclude 13 CMEs having much higher uncertainty from the sample, the agreement between the deflection of CMEs and the ambient field configuration increases substantially, reaching 94% in the new sample of 31 CMEs.

arXiv:1909.06417v1 [pdf, other]
Multi-Particle Collisions in Microgravity: Coefficient of Restitution and Sticking Threshold for Systems of Mm-Sized Particles

The current model of planet formation lacks a good understanding of the growth of dust particles inside the protoplanetary disk beyond mm sizes. In order to investigate the low-velocity collisions between this type of particles, the NanoRocks experiment was flown on the International Space Station (ISS) between September 2014 and March 2016. We present the results of this experiment. We quantify the damping of energy in systems of multiple particles in the 0.1 to 1 mm size range while they are in the bouncing regime, and study the formation of clusters through sticking collisions between particles. We developed statistical methods for the analysis of the large quantity of collision data collected by the experiment. We measured the average motion of particles, the moment of clustering, and the cluster size formed. In addition, we ran simple numerical simulations in order to validate our measurements. We computed the average coefficient of restitution (COR) of collisions and find values ranging from 0.55 for systems including a population of fine grains to 0.94 for systems of denser particles. We also measured the sticking threshold velocities and find values around 1 cm/s, consistent with the current dust collision models based on independently collected experimental data. Our findings have the following implications that can be useful for the simulation of particles in PPDs and planetary rings: (1) The average COR of collisions between same-sized free-floating particles at low speeds (< 2 cm/s) is not dependent on the collision velocity; (2) The simplified approach of using a constant COR value will accurately reproduce the average behavior of a particle system during collisional cooling; (3) At speeds below 5 mm/s, the influence of particle rotation becomes apparent on the collision behavior; (4) Current dust collision models predicting sticking thresholds are robust.

arXiv:1909.06440v1 [pdf, other]
Sub-Kelvin cooling for two kilopixel bolometer arrays in the PIPER receiver

The Primordial Inflation Polarization Explorer (PIPER) is a balloon-borne telescope mission to search for inflationary gravitational waves from the early universe. PIPER employs two 32x40 arrays of superconducting transition-edge sensors, which operate at 100 mK. An open bucket dewar of liquid helium maintains the receiver and telescope optics at 1.7 K. We describe the thermal design of the receiver and sub-kelvin cooling with a continuous adiabatic demagnetization refrigerator (CADR). The CADR operates between 70-130 mK and provides ~10 uW cooling power at 100 mK, nearly five times the loading of the two detector assemblies. We describe electronics and software to robustly control the CADR, overall CADR performance in flight-like integrated receiver testing, and practical considerations for implementation in the balloon float environment.

arXiv:1909.06450v1 [pdf, other]
Magnetospheric Accretion as a Source of H$α$ Emission from Proto-planets around PDS 70
Comments: Accepted for publication in the Astrophysical Journal. 10 pages, 4 figures

Advances in high-resolution imaging have revealed H$\alpha$ emission separated from the host star. It is generally believed that the emission is associated with forming planets in protoplanetary disks. However, the nature of this emission is still not fully understood. Here we report a modeling effort of H$\alpha$ emission from the planets around the young star PDS 70. Using standard magnetospheric accretion models previously applied to accreting young stars, we find that the observed line fluxes can be reproduced using a range of parameters relevant to PDS 70b and c, with the mean mass accretion rate of log(${\rm \dot{M}}$) = $-8.0\pm0.6$ M$_{\rm Jup}$ yr$^{-1}$ and $-8.1\pm0.6$ M$_{\rm Jup}$ yr$^{-1}$ for PDS 70b and PDS 70c, respectively. Our results suggest that H$\alpha$ emission from young planets can originate in the magnetospheric accretion of mass from the circumplanetary disk. We find that empirical relationships between mass accretion rate and H$\alpha$ line properties frequently used in T Tauri stars are not applicable in the planetary mass regime. In particular, the correlations between line flux and mass accretion rate underpredict the accretion rate by about an order of magnitude, and the width at the 10% height of the line is insensitive to the accretion rate at ${\rm \dot{M}}$ $< 10^{-8}$ M$_{\rm Jup}$ yr$^{-1}$.

arXiv:1909.06467v1 [pdf, other]
Cleaning our own Dust: Simulating and Separating Galactic Dust Foregrounds with Neural Networks

Separating galactic foreground emission from maps of the cosmic microwave background (CMB), and quantifying the uncertainty in the CMB maps due to errors in foreground separation are important for avoiding biases in scientific conclusions. Our ability to quantify such uncertainty is limited by our lack of a model for the statistical distribution of the foreground emission. Here we use a Deep Convolutional Generative Adversarial Network (DCGAN) to create an effective non-Gaussian statistical model for intensity of emission by interstellar dust. For training data we use a set of dust maps inferred from observations by the Planck satellite. A DCGAN is uniquely suited for such unsupervised learning tasks as it can learn to model a complex non-Gaussian distribution directly from examples. We then use these simulations to train a second neural network to estimate the underlying CMB signal from dust-contaminated maps. We discuss other potential uses for the trained DCGAN, and the generalization to polarized emission from both dust and synchrotron.

arXiv:1909.06481v1 [pdf, other]
Modeling the Saturation of the Bell Instability Using Hybrid Simulations
Comments: 36th International Cosmic Ray Conference (ICRC2019), held July 24th-August 1st, 2019 in Madison, WI, U.S.A. Online at https://pos.sissa.it/cgi-bin/reader/conf.cgi?confid=358, id.483, 7 pages, 3 figures

The nonresonant cosmic ray instability, predicted by Bell (2004), is thought to play an important role in the acceleration and confinement of cosmic rays (CRs) close to supernova remnants. Despite its importance, the exact mechanism responsible for the saturation of the instability has not been determined, and there is no first-principle prediction for the amplitude of the saturated magnetic field. Using a survey of self-consistent kinetic hybrid simulations (with kinetic ions and fluid electrons), we study the saturation of the non-resonant streaming instability as a function of the parameters of both the thermal background plasma and the CR population. The strength of the saturated magnetic field has important implications for both CR acceleration in supernova remnants and CR diffusion in the Galaxy.

arXiv:1909.06487v1 [pdf, other]
Status of the Keck Planet Imager and Characterizer Phase II Development
Comments: 12 pages; 11 figures; to appear in Proceedings of the SPIE, Techniques and Instrumentation for Detection of Exoplanets IX, Vol. 11117

The Keck Planet Imager and Characterizer comprises of a series of upgrades to the Keck II adaptive optics system and instrument suite to improve the direct imaging and high resolution spectroscopy capabilities of the facility instruments NIRC2 and NIRSPEC, respectively. Phase I of KPIC includes a NIR pyramid wavefront sensor and a Fiber Injection Unit (FIU) to feed NIRSPEC with a single mode fiber, which have already been installed and are currently undergoing commissioning. KPIC will enable High Dispersion Coronagraphy (HDC) of directly imaged exoplanets for the first time, providing potentially improved detection significance and spectral characterization capabilities compared to direct imaging. In favorable cases, Doppler imaging, spin measurements, and molecule mapping are also possible. This science goal drives the development of phase II of KPIC, which is scheduled to be deployed in early 2020. Phase II optimizes the system throughput and contrast using a variety of additional submodules, including a 952 element deformable mirror, phase induced amplitude apodization lenses, an atmospheric dispersion compensator, multiple coronagraphs, a Zernike wavefront sensor, and multiple science ports. A testbed is being built in the Exoplanet Technology Lab at Caltech to characterize and test the design of each of these submodules before KPIC phase II is deployed to Keck. This paper presents an overview of the design of phase II and report on results from laboratory testing.

arXiv:1909.06489v1 [pdf, other]
Aero-Resonant Migration
Comments: 9 pages, 5 figures, accepted to MNRAS

The process of planet conglomeration, which primarily unfolds in a geometrically thin disk of gas and dust, is often accompanied by dynamical excitation of the forming planets and planetesimals. The ensuing orbital crossing can lead to large-scale collisional fragmentation, populating the system with icy and rocky debris. In a gaseous nebula, such leftover solid matter tends to spiral down towards the host star due to aerodynamic drag. Along the way, the inward drifting debris can encounter planets and gravitationally couple to them via mean-motion resonances, sapping them of their orbital energy and causing them to migrate. Here, we develop a simple theory for this migration mechanism, which we call "Aero-Resonant Migration" (ARM), in which small planetesimals ($10\,$m $\lesssim s \lesssim 10\,$km) undergo orbital decay due to aerodynamic drag and resonantly shepherd planets ahead of them. Using a combination of analytical calculations and numerical experiments, we show that ARM is a robust migration mechanism, able to significantly transport planets on timescales $\lesssim1$ Myr, and present simple formulae for the ARM rate.

arXiv:1909.06490v1 [pdf, other]
Relativistic, Viscous, Radiation Hydrodynamic Simulations of Geometrically Thin Disks. II. Disk Variability

We perform detailed variability analysis of two-dimensional viscous, radiation hydrodynamic numerical simulations of Shakura-Sunyaev thin disks around a stellar mass black hole. Disk models are initialized on both the gas-, as well as radiation-, pressure-dominated branches of the thermal equilibrium curve, with mass accretion rates spanning the range from $\dot{M} = 0.01 L_\mathrm{Edd}/c^2$ to $10 L_\mathrm{Edd}/c^2$. An analysis of temporal variations of the numerically simulated disk reveals multiple robust, coherent oscillations. Considering the local mass flux variability, we find an oscillation occurring at the maximum radial epicyclic frequency, $3.5\times 10^{-3}\,t_\mathrm{g}^{-1}$, a possible signature of a trapped fundamental ${\it g}$-mode. Although present in each of our simulated models, the trapped ${\it g}$-mode feature is most prominent in the gas-pressure-dominated case. The total pressure fluctuations in the disk suggest strong evidence for standing-wave ${\it p}$-modes, some trapped in the inner disk close to the ISCO, others present in the middle/outer parts of the disk. Knowing that the trapped ${\it g}$-mode frequency and maximum radial epicyclic frequency differ by only $0.01\%$ in the case of a non-rotating black hole, we simulated an additional initially gas-pressure-dominated disk with a dimensionless black hole spin parameter $a_* = 0.5$. The oscillation frequency in the spinning black hole case confirms that this oscillation is indeed a trapped ${\it g}$-mode. All the numerical models we report here also show a set of high frequency oscillations at the vertical epicyclic and breathing mode frequencies. The vertical oscillations show a 3:2 frequency ratio with oscillations occurring approximately at the radial epicyclic frequency, which could be of astrophysical importance in observed twin peak, high-frequency quasi-periodic oscillations.

arXiv:1909.06566v1 [pdf, other]
Activity time series of old stars from late F to early K IV. Diagnosis from photometry
Comments: Paper published in A&A 629, A42 Some figures have been degraded in resolution compared to the editor version due to Archiv limitations

High-precision time series have recently become available for many stars as a result of data from CoRoT, Kepler, and TESS and have been widely used to study stellar activity. They provide information integrated over the stellar disk, hence many degeneracies between spots and plages or sizes and contrasts. Our aim is to understand how to relate photometric variability to physical parameters in order to help the interpretation of these observations. We computed a large number of synthetic time series of brightness variations for old MS stars within the F6-K4 range, using consistent modeling for radial velocity, astrometry, and LogR'HK. We analyzed these time series to study the effect of the star spectral type on brightness variability, the relationship between brightness variability and LogR'HK, the interpretation of brightness variability as a function of spot and plage properties, and the spot-dominated or plage-dominated regimes. Within our range of activity levels, the brightness variability increases toward low-mass stars, as suggested by Kepler results. Brightness variability roughly correlates to LogR'HK level, but with a large dispersion, caused by spot contrast and inclination. It is also directly related to the number of structures, and we show that it cannot be interpreted solely in terms of spot sizes. In the activity range of old main-sequence stars, we can obtain both spot or plage dominated regimes, as in observation. The same star can be observed in both regimes depending on inclination. Only strong correlations between LogR'HK and brightness variability are significant. Our realistic time series proves to be extremely useful when interpreting observations and understanding their limitations, most notably in terms of activity interpretation. Inclination is crucial and affects many properties, such as amplitudes and the respective role of spots and plages.

arXiv:1909.06580v1 [pdf, other]
Solar Filament Recognition Based on Deep Learning
Comments: 13 pages, 7 figures, 2 tables, accepted for publication in Solar Physics

The paper presents a reliable method using deep learning to recognize solar filaments in H-alpha full-disk solar images automatically. This method cannot only identify filaments accurately but also minimize the effects of noise points of the solar images. Firstly, a raw filament dataset is set up, consisting of tens of thousands of images required for deep learning. Secondly, an automated method for solar filament identification is developed using the U-Net deep convolutional network. To test the performance of the method, a dataset with 60 pairs of manually corrected H-alpha images is employed. These images are obtained from the Big Bear Solar Observatory/Full-Disk H-alpha Patrol Telescope (BBSO/FDHA) in 2013. Cross-validation indicates that the method can efficiently identify filaments in full-disk H-alpha images.

arXiv:1909.06603v1 [pdf, other]
Timing Terminators: Forecasting Sunspot Cycle 25 Onset
Comments: 17 pages, 6 figures, 1 table. Submitted to Solar Physics, Sept 2019

Recent research has demonstrated the existence of a new type of solar event, the "terminator". Unlike the Sun's signature events: flares and Coronal Mass Ejections the terminator takes place in the solar interior. The terminator signals the end of a magnetic activity cycle at the Sun's equator and the start of a sunspot cycle at mid latitudes. Observations indicate that the time difference between these events is very short, less than a solar rotation, in the context of the sunspot cycle. As the (definitive) start and end point of solar activity cycles the precise timing of terminators should permit new investigations into the meteorology of our star's atmosphere. In this letter we use a standard method in signal processing, the Hilbert transform, to identify a mathematically robust signature of terminators in sunspot records and in radiative proxies. Using this technique we can achieve higher fidelity terminator timing than previous estimates have permitted. Further, this method presents a unique opportunity to project when the next terminator will occur, 2020.33(\pm0.16), and trigger the growth of sunspot cycle 25.

arXiv:1909.06637v1 [pdf, other]
First Detection of Plasmoids from Breakout Reconnection on the Sun
Comments: ApJ Letters (in press), 13 pages, 6 figures

Transient collimated plasma ejections (jets) occur frequently throughout the solar corona, in active regions, quiet Sun, and coronal holes. Although magnetic reconnection is generally agreed to be the mechanism of energy release in jets, the factors that dictate the location and rate of reconnection remain unclear. Our previous studies demonstrated that the magnetic breakout model explains the triggering and evolution of most jets over a wide range of scales, through detailed comparisons between our numerical simulations and high-resolution observations. An alternative explanation, the resistive-kink model, invokes breakout reconnection without forming and explosively expelling a flux rope. Here we report direct observations of breakout reconnection and plasmoid formation during two jets in the fan-spine topology of an embedded bipole. For the first time, we observed the formation and evolution of multiple small plasmoids with bidirectional flows associated with fast reconnection in 3D breakout current sheets in the solar corona. The first narrow jet was launched by reconnection at the breakout current sheet originating at the deformed 3D null, without significant flare reconnection or a filament eruption. In contrast, the second jet and release of cool filament plasma were triggered by explosive breakout reconnection when the leading edge of the rising flux rope formed by flare reconnection beneath the filament encountered the preexisting breakout current sheet. These observations solidly support both reconnection-driven jet models: the resistive kink for the first jet, and the breakout model for the second explosive jet with a filament eruption.

arXiv:1909.06688v1 [pdf, other]
Multiple temperature components of the hot circumgalactic medium of the Milky Way
Comments: 13 pages, 5 figures, submitted to ApJL

We present a deep XMM-Newton observation of the Galactic halo emission in the direction of the blazar 1ES 1553+113. In order to extract the Galactic halo component from the diffuse soft X-ray emission spectrum, accurately modeling the foreground components is crucial. Here we present complex modeling of the foregrounds with unprecedented details. A careful analysis of the spectrum yields two temperature components of the halo gas (T$^{em}_1$= 10$^{6.32}$K, T$^{em}_2$= 10$^{6.82}$K). We find that these temperatures obtained from the emission spectrum are not consistent with those from the absorption spectrum (T$^{ab}_1$= 10$^{6.11}$K, T$^{ab}_2$= 10$^{7.06}$K), unlike the previous studies that found only one temperature component of the Milky Way circumgalactic medium. This provides us with interesting insights into the nature of emitting and absorbing systems. We discuss several possibilities objectively, and conclude that most likely we are observing multiple (3 to 4) discrete temperatures between 10$^{5.5}$K and $\geqslant$10$^7$K.

arXiv:1909.06699v1 [pdf, other]
X-shooter spectroscopy of young stars with disks. The TW Hydrae association as a probe of the final stages of disk accretion
Comments: 27 pages (19 in the main text plus Appendices), 7 tables, 19 figures. Accepted for publication in Astronomy & Astrophysics. Abstract shortened for the arXiv listing

We investigate ongoing accretion activity in young stars in the TW Hydrae association (TWA, ~8-10 Myr), an ideal target to probe the final stages of disk accretion down to brown dwarf masses. Our sample comprises eleven TWA members with infrared excess, amounting to 85% of the total TWA population with disks, with spectral types between M0 and M9, and masses between 0.58 and 0.02 Msol. We employed homogeneous spectroscopic data from 300 to 2500 nm, obtained with X-shooter, to derive individual extinction, stellar parameters, and accretion parameters simultaneously. We then examined Balmer lines and forbidden emission lines to probe the physics of the star-disk interaction environment. We detected signatures of ongoing accretion for 70% of our TWA targets. This implies a fraction of accretors of 13-17% across the entire TWA (accounting for potentially accreting members not included in our survey). The spectral emission associated with these stars reveals a more evolved stage of these accretors compared to younger PMS populations: (i) a large fraction (~50%) exhibit nearly symmetric, narrow H_alpha line profiles; (ii) over 80% exhibit Balmer decrements consistent with moderate accretion and optically thin emission; (iii) less than a third exhibit forbidden line emission in [O I] 6300A, indicative of winds and outflows activity. However, the distribution in accretion rates (Mdot) derived for the TWA sample follows closely that of younger regions for Mstar~0.1-0.3 Msun. An overall correlation between Mdot and Mstar is detected, best reproduced by Mdot~Mstar^(2.1+/-0.5). At least in the lowest Mstar regimes, stars that still retain a disk at ages ~8-10 Myr are found to exhibit statistically similar, albeit moderate, accretion levels as those measured around younger objects. This slow Mdot evolution may be associated with longer evolutionary timescales of disks around low-mass stars.

arXiv:1909.06703v1 [pdf, other]
Polarised radiative transfer, rotation measure fluctuations and large-scale magnetic fields
Comments: MNRAS submitted, 19 pages, 10 figures

Faraday rotation measure at radio wavelengths is commonly used to diagnose large-scale magnetic fields. It is argued that the length-scales on which magnetic fields vary in large-scale diffuse astrophysical media can be inferred from correlations in the observed RM. RM is a variable which can be derived from the polarised radiative transfer equations in restrictive conditions. This paper assesses the usage of RMF (rotation measure fluctuation) analyses for magnetic field diagnostics in the framework of polarised radiative transfer. We use models of various magnetic field configurations and electron density distributions to show how density fluctuations could affect the correlation length of the magnetic fields inferred from the conventional RMF analyses. We caution against interpretations of RMF analyses when a characteristic density is ill-defined, e.g. in cases of log-normal distributed and fractal-like density structures. As the spatial correlations are generally not the same in the line-of-sight longitudinal direction and the sky plane direction, one also needs to clarify the context of RMF when inferring from observational data. In complex situations, a covariant polarised radiative transfer calculation is essential to capture all aspects of radiative and transport processes, which would otherwise ambiguate the interpretations of magnetism in galaxy clusters and larger-scale cosmological structures.

arXiv:1909.06712v1 [pdf, other]
A measurement of the Hubble constant from angular diameter distances to two gravitational lenses
Comments: This paper presents the measurements of angular diameter distances to two time-delay lenses, and the Hubble constant derived only from these two distances and the JLA supernova sample. One of the distance measurements is further used for the cosmological inference in the H0LiCOW XIII paper (arxiv:1907.04869). Published in Science

The local expansion rate of the Universe is parametrized by the Hubble constant, $H_0$, the ratio between recession velocity and distance. Different techniques lead to inconsistent estimates of $H_0$. Observations of Type Ia supernovae (SNe) can be used to measure $H_0$, but this requires an external calibrator to convert relative distances to absolute ones. We use the angular diameter distance to strong gravitational lenses as a suitable calibrator, which is only weakly sensitive to cosmological assumptions. We determine the angular diameter distances to two gravitational lenses, $810^{+160}_{-130}$ and $1230^{+180}_{-150}$~Mpc, at redshifts of $z=0.295$ and $0.6304$. Using these absolute distances to calibrate 740 previously-measured relative distances to SNe, we measure the Hubble constant to be $H_0=82.4^{+8.4}_{-8.3} ~{\rm km\,s^{-1}\,Mpc^{-1}}$.

arXiv:1909.06735v1 [pdf, other]
The Radius-Luminosity Relationship Depends on Optical Spectra in Active Galactic Nuclei

The radius-luminosity (R-L) relationship of active galactic nuclei (AGNs) established by the reverberation mapping (RM) observations has been widely used as a single-epoch black hole mass estimator in the research of large AGN samples. However, the recent RM campaigns discovered that the AGNs with high accretion rates show shorter time lags by factors of a few comparing with the predictions from the R-L relationship. The explanation of the shortened time lags has not been finalized yet. We collect 8 different single-epoch spectral properties to investigate how the shortening of the time lags correlate with those properties and to understand what is the origin of the shortened lags. We find that the flux ratio between Fe II and H$\beta$ emission lines shows the most prominent correlation, thus confirm that accretion rate is the main driver for the shortened lags. In addition, we establish a new scaling relation including the relative strength of Fe II emission. This new scaling relation can provide less biased estimates of the black hole mass and accretion rate from the single-epoch spectra of AGNs.

arXiv:1909.06741v1 [pdf, other]
X-ray monitoring of gravitationally lensed radio-loud quasars with Chandra

In this work, we calculated the sizes of unresolved X-ray emission regions in three gravitationally lensed radio-loud quasars, B\,1422+231, MG\,J0414+0534 and Q\,0957+561, using a combination of imaging and spectral analysis on the X-ray data taken from the \textit{Chandra X-Ray Observatory}. We tentatively detected FeK$\alpha$ emission lines in MG\,J0414+0534 and Q\,0957+561 with over 95\% significance, whereas, we did not significantly detect FeK$\alpha$ emission in B\,1422+231. We constructed differential microlensing light curves from absorption corrected count rates. We subsequently performed a microlensing analysis on the X-ray microlensing light curves to measure the X-ray source sizes in soft (0.83--3.6 keV), hard (3.6--21.8 keV), and full (0.83--21.8 keV) bands, based on either Bayesian or maximum likelihood probabilities. For B\,1422+231, sizes from the two methods are consistent with each other, e.g. $R_X^{hard}/R_G = 6.17\pm5.48 \text{ (Bayesian), } 11.81\pm3.75 \text{ (maximum likelihood)}$, where $R_G=GM_{BH}/c^2)$. However, for MG\,J0414+0534 and Q\,0957+561, the two methods yield completely different results suggesting that more frequently sampled data with better signal-to-noise ratio are needed to measure the source size for these two objects. Comparing the acquired size values with the radio-quiet sample in the literature we found that our results are consistent with X-ray source size scaling approximately as $R_X \propto M_{BH}$ with the mass of the central supermassive black hole. Our results also indicate that radio-loud quasars tend to have larger unresolved X-ray emission sizes compared to the radio-quiet ones.

arXiv:1909.06745v1 [pdf, other]
Investigating the gas-to-dust ratio in the protoplanetary disk of HD 142527

We present ALMA observations of the $98.5~\mathrm{GHz}$ dust continuum and the $\mathrm{^{13}CO}~J = 1 - 0$ and $\mathrm{C^{18}O}~J = 1 - 0$ line emissions of the protoplanetary disk associated with HD~142527. The $98.5~\mathrm{GHz}$ continuum shows a strong azimuthal-asymmetric distribution similar to that of the previously reported $336~\mathrm{GHz}$ continuum, with a peak emission in dust concentrated region in the north. The disk is optically thin in both the $98.5~\mathrm{GHz}$ dust continuum and the $\mathrm{C^{18}O}~J = 1 - 0$ emissions. We derive the distributions of gas and dust surface densities, $\Sigma_\mathrm{g}$ and $\Sigma_\mathrm{d}$, and the dust spectral opacity index, $\beta$, in the disk from ALMA Band 3 and Band 7 data. In the analyses, we assume the local thermodynamic equilibrium and the disk temperature to be equal to the peak brightness temperature of $\mathrm{^{13}CO}~J = 3 - 2$ with a continuum emission. The gas-to-dust ratio, $\mathrm{G/D}$, varies azimuthally with a relation $\mathrm{G/D} \propto \Sigma_\mathrm{d}^{-0.53}$, and $\beta$ is derived to be $\approx 1$ and $\approx 1.7$ in the northern and southern regions of the disk, respectively. These results are consistent with the accumulation of larger dust grains in a higher pressure region. In addition, our results show that the peak $\Sigma_\mathrm{d}$ is located ahead of the peak $\Sigma_\mathrm{g}$. If the latter corresponds to a vortex of high gas pressure, the results indicate that the dust is trapped ahead of the vortex, as predicted by some theoretical studies.

arXiv:1909.06748v1 [pdf, other]
Planet Formation around Super Massive Black Holes in the Active Galactic Nuclei
Comments: 14 pages, 4 figures, submitted to the Asrophysical Journal. Comments are welcome

As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line'', located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier'' does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1--1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10--100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with ten times the mass of the earth. The growth time scale depends on the turbulent strength of the circumnuclear disk and the black hole mass $M_{BH}$, and it is comparable to the AGN's lifetime ($\sim 10^8$ yr) for low mass ($M_{BH} \sim 10^6 M_\odot$) SMBHs.

arXiv:1909.06801v1 [pdf, other]
X-ray spectral variations of synchrotron peak in BL Lacs
Comments: 19 pages, 10 figures, 2 tables; Accepted for publication in ApJ

The spectral energy distribution of blazars around the synchrotron peak can be well described by the log-parabolic model that has three parameters: peak energy ($E_\textrm{p}$), peak luminosity ($L_\textrm{p}$) and the curvature parameter ($b$). It has been suggested that $E_\textrm{p}$ shows relations with $L_\textrm{p}$ and $b$ in several sources, which can be used to constrain the physical properties of the emitting region and/or acceleration processes of the emitting particles. We systematically study the $E_\textrm{p}$-$L_\textrm{p}$ and $E_\textrm{p}$-(1$/b$) relations for 14 BL Lac objects using the 3-25~keV $RXTE$/PCA and 0.3-10~keV $Swift$/XRT data. Most objects (9/14) exhibit positive $E_\textrm{p}$-$L_\textrm{p}$ correlations, three sources show no correlation, and two sources display negative correlations. In addition, most targets (7/14) present no correlation between $E_\textrm{p}$ and 1$/b$, five sources pose negative correlations, and two sources demonstrate positive correlations. 1ES~1959+650 displays two different $E_\textrm{p}$-$L_\textrm{p}$ relations in 2002 and 2016. We also analyze $E_\textrm{p}$-$L_\textrm{p}$ and $E_\textrm{p}$-(1$/b$) relations during flares lasting for several days. The $E_\textrm{p}$-$L_\textrm{p}$ relation does not exhibit significant differences between flares, while the $E_\textrm{p}$-(1$/b$) relation varies from flare to flare. For the total sample, when $L_\textrm{p}$ < $\textrm{10}^\textrm{45}\ \textrm{erg}\ \textrm{s}^\textrm{-1}$, there seems to be a positive $E_\textrm{p}$-$L_\textrm{p}$ correlation. $L_\textrm{p}$ and the slope of $E_\textrm{p}$-$L_\textrm{p}$ relation present an anti-correlation, which indicates that the causes of spectral variations might be different between luminous and faint sources. $E_\textrm{p}$ shows a positive correlation with the black hole mass. We discuss the implications of these results.

arXiv:1909.06828v1 [pdf, other]
The relationship between flux emergence and subsurface toroidal magnetic flux

The 1-D mean-field equation describing the evolution of the subsurface toroidal field can be used together with the observed surface radial field to model the subsurface toroidal flux density. We aim to test this model and determine the relationship between the observationally inferred surface toroidal field (as a proxy for flux emergence), and the modelled subsurface toroidal flux density. We use a combination of sunspot area observations, the surface toroidal field inferred from WSO line-of-sight magnetic field observations, and compare with the results of a 1-D mean-field evolution equation for the subsurface toroidal field, driven by the observed radial field from the National Solar Observatory/Kitt Peak and SOLIS observations. We derive calibration curves relating the subsurface toroidal flux density to the observed surface toroidal field strengths and sunspot areas. The calibration curves are for two regimes, one corresponding to ephemeral region emergence outside of the butterfly wings, the other to active region emergence in the butterfly wings. We discuss this in terms of the size and vertical velocity associated with the two types of flux emergence.

arXiv:1909.06830v1 [pdf, other]
Monte Carlo variations as a tool to assess nuclear physics uncertainties in nucleosynthesis studies
Comments: 6 pages, 1 figure; Proceedings of the 27th International Nuclear Physics Conference, Glasgow, UK, 2019; to appear in IOP Conf. Proc

The propagation of uncertainties in reaction cross sections and rates of neutron-, proton-, and $\alpha$-induced reactions into the final isotopic abundances obtained in nucleosynthesis models is an important issue in studies of nucleosynthesis and Galactic Chemical Evolution. We developed a Monte Carlo method to allow large-scale postprocessing studies of the impact of nuclear uncertainties on nucleosynthesis. Temperature-dependent rate uncertainties combining realistic experimental and theoretical uncertainties are used. From detailed statistical analyses uncertainties in the final abundances are derived as probability density distributions. Furthermore, based on rate and abundance correlations an automated procedure identifies the most important reactions in complex flow patterns from superposition of many zones or tracers. The method already has been applied to a number of nucleosynthesis processes.

arXiv:1909.06843v1 [pdf, other]
Complex Organic Molecules in Star-Forming Regions of the Magellanic Clouds
Comments: This document is unedited Author's version of a Submitted Work that was subsequently accepted for publication in ACS Earth and Space Chemistry, copyright American Chemical Society, after peer review. To access the final edited and published work, see https://pubs.acs.org/articlesonrequest/AOR-mPUVfkWhtGmqXI5KbiR7

The Large and Small Magellanic Clouds (LMC and SMC), gas-rich dwarf companions of the Milky Way, are the nearest laboratories for detailed studies on the formation and survival of complex organic molecules (COMs) under metal poor conditions. To date, only methanol, methyl formate, and dimethyl ether have been detected in these galaxies - all three toward two hot cores in the N113 star-forming region in the LMC, the only extragalactic sources exhibiting complex hot core chemistry. We describe a small and diverse sample of the LMC and SMC sources associated with COMs or hot core chemistry, and compare the observations to theoretical model predictions. Theoretical models accounting for the physical conditions and metallicity of hot molecular cores in the Magellanic Clouds have been able to broadly account for the existing observations, but fail to reproduce the dimethyl ether abundance by more than an order of magnitude. We discuss future prospects for research in the field of complex chemistry in the low-metallicity environment. The detection of COMs in the Magellanic Clouds has important implications for astrobiology. The metallicity of the Magellanic Clouds is similar to galaxies in the earlier epochs of the Universe, thus the presence of COMs in the LMC and SMC indicates that a similar prebiotic chemistry leading to the emergence of life, as it happened on Earth, is possible in low-metallicity systems in the earlier Universe.

arXiv:1909.06850v1 [pdf, other]
A nearby recent supernova that ejected the runaway star zeta Oph, the pulsar PSRB1706-16, and 60-Fe found on Earth
Comments: 15 pages with 6 tables and 6 figures, accepted for publication in MNRAS

The detection of $\sim 1.5-3.2$ Myr old $^{60}$Fe on Earth indicates recent nearby core-collapse supernovae. For supernovae in multiple stars, the primary stars may become neutron stars, while former companions may become unbound and become runaway stars. We wrote software for tracing back the space motion of runaway and neutron stars to young associations of massive stars. We apply it here to the nearby young Scorpius-Centaurus-Lupus groups, all known runaway stars possibly coming from there, and all 400 neutron stars with known transverse velocity. We find kinematic evidence that the runaway $\zeta$ Oph and the radio pulsar PSRB1706-16 were released by a supernova in a binary $1.78 \pm 0.21$ Myr ago at $107 \pm 4$ pc distance (for pulsar radial velocity $260 \pm 43$ km/s); association age and flight time determine the progenitor mass (16-18 M$_{\odot}$), which can constrain supernova nucleosynthesis yields and $^{60}$Fe uptake on Earth. In addition, we notice that the only high-mass X-ray binary in Scorpius-Centaurus-Lupus (1H11255-567 with $\mu^{1}$ and $\mu^{2}$ Cru) may include a neutron star formed in another SN, up to $\sim 1.8$ Myr ago at $89-112$ pc, i.e. also yielding $^{60}$Fe detectable on Earth. Our scenario links $^{60}$Fe found on Earth to one or two individual supernovae in multiple stars.

arXiv:1909.06931v1 [pdf, other]
Three-dimensional simulations of non-resonant streaming instability and particle acceleration near non-relativistic astrophysical shocks
Comments: Accepted for publication in MNRAS The published version will contain an animation of the 3-D simulation

We use particle-in-magnetohydrodynamics-cells to model particle acceleration and magnetic field amplification in a high Mach, parallel shock in three dimensions and compare the result to 2-D models. This allows us to determine whether 2-D simulations can be relied upon to yield accurate results in terms of particle acceleration, magnetic field amplification and the growth rate of instabilities. Our simulations show that the behaviour of the gas and the evolution of the instabilities are qualitatively similar for both the 2-D and 3-D models, with only minor quantitative differences that relate primarily to the growth speed of the instabilities. The main difference between 2-D and 3-D models can be found in the spectral energy distributions (SEDs) of the non-thermal particles. The 2-D simulations prove to be more efficient, accelerating a larger fraction of the particles and achieving higher velocities. We conclude that, while 2-D models are sufficient to investigate the instabilities in the gas, their results have to be treated with some caution when predicting the expected SED of a given shock.

arXiv:1909.06944v1 [pdf, other]
The equation of state and some key parameters of neutron stars: constraints from GW170817, the nuclear data and the low mass X-ray binary data
Comments: 12 pages including 5 figures, ApJ in press

In this work we parameterize the Equation of State of dense neutron star (NS) matter with four pressure parameters of $\{\hat{p}_1, \hat{p}_2, \hat{p}_3, \hat{p}_4\}$ and then set the combined constraints with the data of GW 170817 and the data of 6 Low Mass X-ray Binaries (LMXBs) with thermonuclear burst or alternatively the symmetry energy of the nuclear interaction. We find that the nuclear data effectively narrow down the possible range of $\hat{p}_1$, the gravitational wave data plays the leading role in bounding $\hat{p}_2$, and the LMXB data as well as the lower bound on maximal gravitational mass of non-rotating NSs govern the constraints on $\hat{p}_3$ and $\hat{p}_4$. Using posterior samples of pressure parameters and some universal relations, we further investigate how the current data sets can advance our understanding of tidal deformability ($\Lambda$), moment of inertia ($I$) and binding energy ($BE$) of NSs. For a canonical mass of $1.4M_\odot$, we have $I_{1.4} = {1.43}^{+0.30}_{-0.13} \times 10^{38}~{\rm kg \cdot m^2}$, $\Lambda_{1.4} = 390_{-210}^{+280}$ , $R_{1.4} = 11.8_{-0.7}^{+1.2}~{\rm km}$ and $BE_{1.4} = {0.16}^{+0.01}_{-0.02} M_{\odot}$ if the constraints from the nuclear data and the gravitational wave data have been jointly applied. For the joint analysis of gravitational wave data and the LMXB data, we have $I_{1.4} = {1.28}^{+0.15}_{-0.08} \times 10^{38}~{\rm kg \cdot m^2}$, $\Lambda_{1.4} = 220_{-90}^{+90}$, $R_{1.4} = 11.1_{-0.6}^{+0.7}~{\rm km}$ and $BE_{1.4} = {0.18}^{+0.01}_{-0.01} M_{\odot}$. These results suggest that the current constraints on $\Lambda$ and $R$ still suffer from significant systematic uncertainties, while $I_{1.4}$ and $BE_{1.4}$ are better constrained.

arXiv:1909.07001v1 [pdf, other]
Rotational disruption of dust grains by high-velocity gas-grain collisions

Dust grains moving at hypersonic velocities of $v_{d}\gtrsim 100~\rm km~s^{-1}$ through an ambient gas are known to be destroyed efficiently by nonthermal sputtering. Yet, previous studies of nonthermal sputtering disregarded the fact that the grain can be spun-up to suprathermal rotation by stochastic gas-grain collisions. In this paper, we show that such a suprathermal rotation results in the disruption of the small grain into molecules because the induced centrifugal stress can exceed the maximum tensile strength of grain material, $S_{\rm max}$. We term this mechanism {\it rotational disruption}. We find that rotational disruption is more efficient than nonthermal sputtering in destroying small dust grains of nonideal internal structures moving with velocities of $v_{d}>100 ~\rm km~s^{-1}$. The ratio of rotational disruption to sputtering time is $\tau_{\rm disr}/\tau_{sp}\sim 0.2(S_{\rm max}/10^{9}\rm erg~cm^{-3})(Y_{sp}/0.1)(a/0.01\mu m)^{3}(300~\rm km~s^{-1}/v_{d})^{2}$ where $a$ is the radius of spherical grains, and $Y_{sp}$ is the sputtering yield. We discuss the implication of this mechanism for the destruction of hypersonic grains accelerated by radiation pressure as well as grains in fast shocks. Our results suggest that the destruction of dust grains in fast shocks of supernova remnants is more efficient than previously predicted by sputtering, which seems to be supported by the higher fraction of dust destruction observed in fast shocks of core-collapse supernovae.

arXiv:1909.07008v1 [pdf, other]
Fine-grained rim formation -- high speed, kinetic dust aggregation in the early Solar System

Type 3 chondritic meteorites often contain chondrules and refractory inclusions that are coated with accretionary, fine-grained rims (FGRs). FGRs are of low porosity, were subject to centrally directed pressure, may contain high temperature products like microchondrules and there is a linear relationship between the rim thickness and the radius of the enclosed object. FGRs are thought to have formed by the gentle adhesion of dust onto the central object with the subsequent compression of this fluffy rim within the parent body. However, this model does not explain the low porosity, micro-chondrules and centralized pressure. This model also has difficulties explaining the linear relationship between rim thickness and object size including the existence of a non-zero constant in that linear relationship. We propose that FGRs formed by the relatively high-speed interaction between dust and the object, where high initial impact speed produced abrasion and, possibly, microchondrules. FGR formation occurred over a range of lower speeds aided by vacuum adhesion of fragments from the impacting dust particles. This model naturally produces the rim thickness linear relationship with non-zero constant, low porosity and centrally directed pressure. We call this process kinetic dust aggregation (KDA), which is another name for the aerosol deposition processes used in industry. KDA may be a tentative, part explanation of how dust aggregation occurs in protostellar disks on the pathway from dust to planets.

arXiv:1909.07011v1 [pdf, other]
The impact of (n,$γ$) reaction rate uncertainties on the predicted abundances of i-process elements with $32\leq Z\leq 48$ in the metal-poor star HD94028
Comments: 10 pages, 14 figures, submitted to MNRAS

Several anomalous elemental abundance ratios have been observed in the metal-poor star HD94028. Following Roederer et al. (2016), we assume that its high [As/Ge] ratio is a product of a weak intermediate (i) neutron-capture process. Given that observational errors are usually smaller than predicted nuclear physics uncertainties, we have first set up a benchmark one-zone i-process nucleosynthesis simulation results of which provide the best fit to the observed abundances. We have then performed Monte Carlo simulations in which 113 relevant (n,$\gamma$) reaction rates of unstable species were randomly varied within Hauser-Feshbach model uncertainty ranges for each reaction to estimate the impact on the predicted stellar abundances. One of the interesting results of these simulations is a double-peaked distribution of the As abundance, which is caused by the variation of the $^{75}$Ga (n,$\gamma$) cross section. This variation strongly anti-correlates with the predicted As abundance, confirming the necessity for improved theoretical or experimental bounds on this cross section. The $^{66}$Ni (n,$\gamma$) reaction is found to behave as a major bottleneck for the i-process nucleosynthesis. Our analysis finds the Pearson product-moment correlation coefficient $r_\mathrm{P} > 0.2$ for all of the i-process elements with $32 \leq Z \leq 42$, with significant changes in their predicted abundances showing up when the rate of this reaction is reduced to its theoretically constrained lower bound. Our results are applicable to any other stellar nucleosynthesis site with the similar i-process conditions, such as Sakurai's object (V4334 Sagittarii) or rapidly-accreting white dwarfs.

arXiv:1909.07014v1 [pdf, other]
The W-subtype active contact binary PZ UMa with a possible more massive tertiary component
Comments: 13 pages, 6 figures, published in PASJ

Two sets of multiple-color ($B, V, R_c, I_c$) light curves of PZ UMa were observed in dependently with the 2.4 meter telescope at the Thai National Observatory and the 1 meter telescope at Yunnan Observatories. The light curves were analyzed with the Wilson-Devinney program and the two sets of light curves produced consistent results, which show that PZ UMa is a W-subtype contact binary with an extreme mass ratio ($M_{1}/M_{2} = 0.18)$. The basic physical parameters of PZ UMa were determined to be $M_{2} = 0.77(2)M_\odot$, $M_{1} = 0.14(1)M_\odot$, $R_{2} = 0.92(1)R_\odot$, $R_{1} = 0.43(1)R_\odot$, $L_{2} = 0.46(2)L_\odot$ and $L_{1} = 0.15(3)L_\odot$. The orbital period analysis of PZ UMa revealed a 13.22 year periodicity, which implies that there may be a tertiary component orbiting around the binary system. The mass and orbital radius of the tertiary component were calculated to be $M_{3} = 0.88 M_\odot$ and $a_{3} = 3.67 AU$, if the orbit was coplanar with the central binary system. It is interesting that the minimum mass of the tertiary was calculated to be $M_{3min} = 0.84 M_\odot$, which means the tertiary component is even larger than the primary star and the secondary one of PZ UMa. PZ UMa is a late-type contact binary with stellar activity. The O'Connell effect appeared on its light curves when it was observed on April 2016. However, the O'Connell effect reversed when the target was observed again on December 2016. The changes of the O'Connell effect in such a short time-scale strongly support the occurrence of rapidly changing magnetic activity on this W UMa binary.

arXiv:1909.07034v1 [pdf, other]
Discrimination of heavy elements originating from Pop III stars in z = 3 intergalactic medium
Comments: 9 pages, 8 figures, submitted to MNRAS

We investigate the distribution of metals in the cosmological volume at $z\sim3$, in particular, provided by massive population III (Pop. III) stars using a cosmological $N$-body simulation in which a model of Pop. III star formation is implemented. Owing to the simulation, we can choose minihaloes where Pop. III star formation occurs at $z>10$ and obtain the spatial distribution of the metals at lower-redshifts. To evaluate the amount of heavy elements provided by Pop. III stars, we consider metal yield of pair-instability or core-collapse supernovae (SNe) explosions of massive stars. By comparing our results to the Illustris-1 simulation, we find that heavy elements provided by Pop. III stars often dominate those from galaxies in low density regions. The median value of the volume averaged metallicity is $Z\sim 10^{-4.5 - -2} Z_{\odot}$ at the regions. We also show that spectroscopic observations with the next generation telescopes are expected to detect the metals imprinted on quasar spectra.

arXiv:1909.07035v1 [pdf, other]
Study of the lateral distribution functions of electron and muon bundles using Trasgo detectors

Some of the main features of the new generation Trasgo detectors are their capability in measuring the incoming direction and the arrival time of secondary cosmic particles. They also offer the identification capability between muon and electrons and a rough calorimetry for electrons. Using ground-based stations, these properties allow for the development of new tools for the measurement of primary cosmic ray fluxes. In order to verify and quantify the suitability of Trasgo detectors, whether a single one or arrays of them, to provide reliable information of the properties (mass, energy, incoming direction) of primary cosmic rays we have started an initiative for the systematic study of the 'lateral distributions' displayed by electrons and muons, or by bundles of those particles, using MonteCarlo simulations. In a first approach, electrons and muons were produced in vertical showers from primary H, He, C and Fe nuclei, and with incoming energies limited to a maximum of 10$^{15}$ eV per nucleon. This choice represents a significant component of all secondary particles, which can be measured on Earth's surface. The lateral distributions study has been done at the two locations of Santiago de Compostela (Spain) and Livingston Island (Antarctica), where Trasgo detectors are either in operation, or will be operative in the near future.

arXiv:1909.07044v1 [pdf, other]
Flare Activity and Magnetic Feature Analysis of the Flare Stars

We analyze the light curve of 1740 flare stars to study the relationship between the magnetic feature characteristics and the identified flare activity. Coverage and stability of magnetic features are inspired by rotational modulation of light curve variations and flare activity of stars are obtained using our automated flare detection algorithm. The results show that (i) Flare time occupation ratio (or flare frequency) and total power of flares increase by increasing relative magnetic feature coverage and contrast in F-M type stars (ii) Magnetic feature stability is highly correlated with the coverage and the contrast of the magnetic structures as this is the case for the Sun (iii) Stability, coverage and contrast of the magnetic features, time occupation ratio and total power of flares increases for G, K and M-type stars by decreasing Rossby number due to the excess of produced magnetic field from dynamo procedure until reaching to saturation level.

arXiv:1909.07103v1 [pdf, other]
The mass dependence of structure, star formation rate, and mass assembly mode at 0.5<z< 2.5
Comments: 19 pages, 10 figures, accepted by ApJ

To investigate the mass dependence of structural transformation and star formation quenching, we construct three galaxy samples using massive ($M_* > 10^{10} M_{\odot}$) red, green, and blue galaxy populations at $0.5<z<2.5$ in five 3D--{\it HST}/CANDELS fields. The structural parameters, including effective radius ($r_{\rm e}$), galaxy compactness ($\Sigma_{1.5}$), and second order moment of 20\% brightest pixels ($M_{20}$) are found to be correlated with stellar mass. S\'{e}rsic index ($n$), concentration ($C$), and Gini coefficient ($G$) seem to be insensitive to stellar mass. The morphological distinction between blue and red galaxies is found at a fixed mass bin, suggesting that quenching processes should be accompanied with transformations of galaxy structure and morphology. Except for $r_e$ and $\Sigma_{1.5}$ at high mass end, structural parameters of green galaxies are intermediate between red and blue galaxies in each stellar mass bin at $z < 2$, indicating green galaxies are at a transitional phase when blue galaxies are being quenched into quiescent statuses. The similar sizes and compactness for the blue and green galaxies at high-mass end implies that these galaxies will not appear to be significantly shrunk until they are completely quenched into red QGs. For the green galaxies at $0.5<z<1.5$, a morphological transformation sequence of bulge buildup can be seen as they are gradually shut down their star formation activities, while a faster morphological transformation is verified for the green galaxies at $1.5<z<2.5$.

arXiv:1909.07118v1 [pdf, other]
The Inside-out Growth of the Galactic Disk

We quantify the inside-out growth of the Milky Way's low-alpha stellar disk, modelling the ages, metallicities and Galactocentric radii of APOGEE red clump stars with 6 < R < 13 kpc. The current stellar distribution differs significantly from that expected from the star formation history due to the redistribution of stars through radial orbit mixing. We propose and fit a global model for the Milky Way disk, specified by an inside-out star formation history, radial orbit mixing, and an empirical, parametric model for its chemical evolution. We account for the spatially complex survey selection function, and find that the model fits all data well. We find distinct inside-out growth of the Milky Way disk; the best fit model implies that the half-mass radius of the Milky Way disk has grown by 43\% over the last 7 Gyr. Yet, such inside-out growth still results in present-day age gradient weaker than 0.1 Gyr/kpc. Our model predicts the half-mass and half-light sizes of the Galactic disk at earlier epochs, which can be compared to the observed redshift -size relations of disk galaxies. We show that radial orbit migration can reconcile the distinct disk-size evolution with redshift, also expected from cosmological simulations, with the modest present-day age gradients seen in the Milky Way and other galaxies.

arXiv:1909.07132v1 [pdf, other]
Evolving neutron star+helium star systems to intermediate-mass binary pulsars
Comments: accepted for publication in MNRAS

Intermediate-mass binary pulsars (IMBPs) are composed of neutron stars (NSs) and CO/ONe white dwarfs (WDs). It is generally suggested that IMBPs evolve from intermediate-mass X-ray binaries (IMXBs). However, this scenario is difficult to explain the formation of IMBPs with orbital periods ($P_{\rm orb}$) less than 3 d. It has recently been proposed that a system consisting of a neutron star (NS) and a helium (He) star can form IMBPs with $P_{\rm orb}$ less than 3 d (known as the NS+He star scenario), but previous works can only cover a few observed sources with short orbital periods. We aim to investigate the NS+He star scenario by adopting different descriptions of the Eddington accretion rate ($\dot M_{\rm Edd}$) for NSs and different NS masses ($M_{\rm NS}$) varying from $1.10\,\rm M_{\odot}$ to $1.80\,\rm M_{\odot}$. Our results can cover most of the observed IMBPs with short orbital periods and almost half of the observed IMBPs with long orbital periods. We found that $\dot{M}_{\rm Edd}$$\propto$$M_{\rm NS}$$^{\rm -1/3}$ could match the observations better than a specific value for all NSs. We also found that the final spin periods of NSs slightly decrease with the initial $M_{\rm NS}$. The observed parameters of PSR J0621+1002, which is one of the well-observed IMBPs whose pulsar mass has been precisely measured, can be reproduced by the present work.

arXiv:1909.07136v1 [pdf, other]
New Results from the Cosmic-Ray Program of the NA61/SHINE facility at the CERN SPS
Comments: To appear on the proceedings of the 36th International Cosmic Ray Conference (ICRC 2019)

The NA61/SHINE experiment at the SPS accelerator at CERN is a unique facility for the study of hadronic interactions at fixed target energies. The data collected with NA61/SHINE is relevant for a broad range of topics in cosmic-ray physics including ultrahigh-energy air showers and the production of secondary nuclei and anti-particles in the Galaxy. Here we present an update of the measurement of the momentum spectra of anti-protons produced in $\pi^-$+C interactions at 158 and 350 GeV/c and discuss their relevance for the understanding of muons in air showers initiated by ultrahigh-energy cosmic rays. Furthermore, we report the first results from a three-day pilot run aimed at investigating the capability of our experiment to measure nuclear fragmentation cross sections for the understanding of the propagation of cosmic rays in the Galaxy. We present a preliminary measurement of the production cross section of Boron in C+p interactions at 13.5 AGeV/c and discuss prospects for future data taking to provide the comprehensive and accurate reaction database of nuclear fragmentation needed in the era of high-precision measurements of Galactic cosmic rays.

arXiv:1909.07148v1 [pdf, other]
Discovery of an accretion-rate independent absolute RMS amplitude of millihertz quasi-periodic oscillations in 4U 1636-53
Comments: Accepted for publication in ApJ, 9 pages, 8 figures

We investigate the frequency and amplitude of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary (NS LMXB) 4U 1636-53 using Rossi X-ray Timing Explorer observations. We find that no mHz QPOs appear when the source is in the hard spectral state. We also find that there is no significant correlation between the frequency and the fractional rms amplitude of the mHz QPOs. Notwithstanding, for the first time, we find that the absolute RMS amplitude of the mHz QPOs is insensitive to the parameter Sa, which measures the position of the source in the colour-colour diagram and is usually assumed to be an increasing function of mass accretion rate. This finding indicates that the transition from marginally stable burning to stable burning or unstable burning could happen very rapidly since, before the transition, the mHz QPOs do not gradually decay as the rate further changes.

arXiv:1909.07150v1 [pdf, other]
Numerical models for the dust in RCW 120
Comments: 6 pages, 5 figures, 1 table, accepted for publication in A&A

The interstellar bubble RCW 120 seen around a type O runaway star is driven by the stellar wind and the ionising radiation emitted by the star. The boundary between the stellar wind and interstellar medium (ISM) is associated with the arc-shaped mid-infrared dust emission around the star within the HII region.

arXiv:1909.07165v1 [pdf, other]
A cumulative search for hard X/$γ$-Ray emission associated with fast radio bursts in Fermi/GBM data
Comments: 8 pages, 6 figures, Accepted on A&A

Context. Fast Radio Bursts (FRBs) are millisecond-long bursts uniquely detected at radio frequencies. FRB 131104 is the only case for which a $\gamma$-ray transient positionally and temporally consistent was claimed. This high-energy transient had a duration of $\sim400$~s and a 15-150~keV fluence $S_{\gamma}\sim4\times10^{-6}$ erg $\mathrm{cm}^{-2}$. However, the association with the FRB is still debated. Aims. We aim at testing the systematic presence of an associated transient high-energy counterpart throughout a sample of the FRB population. Methods. We used an approach like that used in machine learning methodologies to accurately model the highly-variable Fermi/GBM instrumental background on a time interval comparable to the duration of the proposed $\gamma$-ray counterpart of FRB 131104. A possible $\gamma$-ray signal is then constrained considering sample average lightcurves. Results. We constrain the fluence of the possible $\gamma$-ray signal in the 8-1000 keV band down to $6.4 \times 10^{-7}$ ($7.1 \times 10^{-8}$) erg cm$^{-2}$ for a 200-s (1-s) integration time. Furthermore, we found the radio-to-gamma fluence ratio to be $\eta>10^{8}$ Jy ms erg$^{-1}$ cm$^2$. Conclusions. Our fluence limits exclude $\sim 94\%$ of Fermi/GBM detected long gamma-ray bursts and $\sim 96\%$ of Fermi/GBM detected short gamma-ray bursts. In addition, our limits on the radio-to-gamma fluence ratio point to a different emission mechanism from that of magnetar giant flares. Finally, we exclude a $\gamma$-ray counterpart as fluent as the one possibly associated with FRB 131104 to be a common feature of FRBs.

arXiv:1909.07174v1 [pdf, other]
Unfolding the True Atmospheric Neutrino Event Rate in the 1Gev -- 1Tev Range Using IceCube/DeepCore
Comments: Presented at the 36th International Cosmic Ray Conference (ICRC 2019). See arXiv:1907.11699 for all IceCube contributions

An unfolding measurement of the atmospheric neutrino flux at the South Pole is performed using the IceCube/DeepCore detector. The main results presented is the true neutrino interaction rate by unit volume of ice, as a function of energy or zenith angle. The method used is the D'Agostini iterative unfolding. While the detector response estimate is based on Monte Carlo simulation, the iterative approach will compensate for this inherent bias and draw the unfolded result closer to the unbiased estimator, as the number of iterations is optimized. This is done using an ensemble test with a blind re-smearing approach. Thus this measurement is minimally biased regarding atmospheric flux-, oscillation- and neutrino interaction models, allowing model builders to test their predictions against this measurement. As an aside, using the same data set and methodology, we also present an unfolding measurement directly to atmospheric neutrino flux.

arXiv:1909.07183v1 [pdf, other]
VTC J095517.5+690813: A radio transient from an accretion-induced collapse of a white dwarf?
Comments: 6 pages, 3 figures, 1 table, accepted by Monthly Notices of the Royal Astronomical Society

We investigate a possibility that a recently reported radio transient in M81, VTC J095517.5+690813, is caused by an accretion-induced collapse of a white dwarf. It became bright in radio but no associated optical transient was discovered. The accretion-induced collapse is predicted to be radio bright but optically faint, satisfying the observed property. We compare predicted radio emission from the accretion-induced collapse with that of VTC J095517.5+690813 and show that it can be reasonably explained by the accretion-induced collapse. Although it is difficult to firmly conclude that VTC J095517.5+690813 is an accretion-induced collapse, our study shows that radio-bright transients without an optical counterpart could still be related to stellar deaths.

arXiv:1909.07217v1 [pdf, other]
Lower tensor to scalar ratio in a SUGRA motivated inflationary potential

A scalar potential obtained from the $D$-term in the Supergravity models, responsible for the inflationary phase in the early universe, is studied. The potential has a very slow roll feature in comparison to various other plateau type inflationary potentials. Thus, a much lower tensor-to-scalar ratio is obtained in this case. The predicted values of the inflationary observables are well within the 1-$\sigma$ bounds of the recent constraints from Planck'18 observations. The era of reheating after the inflationary phase is also studied and the bounds on the reheating temperature ($T_{re}$) is calculated for different equation of states during reheating ($w_{re}$) for the Planck'18 allowed values of the scalar spectral index ($n_s$).

arXiv:1909.07221v1 [pdf, other]
A simple two-component description of mass segregation for anisotropic globular clusters
Comments: 4 pages, 2 figure, to appear in Proceedings IAU Symposium No. 351 Star Clusters: From the Milky Way to the Early Universe, 2019

As a result of the slow action of two-body encounters, globular clusters develop mass segregation and attain a condition of only partial energy equipartition even in their central, most relaxed regions. Realistic numerical simulations show that, during the process, a radially-biased anisotropy profile slowly builds up, mimicking that resulting from incomplete violent relaxation. Commonly used dynamical models, such as the one-component King models, cannot describe these properties. Here we show that simple two-component models based on a distribution function originally conceived to describe elliptical galaxies, recently truncated and adapted to the context of globular clusters, can describe in detail what is observed in complex and realistic numerical simulations.

arXiv:1909.07225v1 [pdf, other]
Presolar Grains in Primitive Ungrouped Carbonaceous Chondrite Northwest Africa 5958
Comments: Accepted in Meteoritics and Planetary Science (32 pages, 9 figures). In accordance with Wiley license restrictions, the submitted version is provided. Dedicated to the memory of Dr. Christine Floss

We report a correlated NanoSIMS-transmission electron microscopy study of the ungrouped carbonaceous chondrite Northwest Africa (NWA) 5958. We identified 10 presolar SiC grains, 2 likely presolar graphite grains, and 20 presolar silicate and/or oxide grains in NWA 5958. We suggest a slight modification of the commonly used classification system for presolar oxides and silicates that better reflects the grains' likely stellar origins. The matrix-normalized presolar SiC abundance in NWA 5958 is $18_{-10}^{+15}$ ppm (2$\sigma$), similar to that seen in many classes of unmetamorphosed chondrites. In contrast, the matrix-normalized abundance of presolar O-rich phases (silicates and oxides) is $30.9_{-13.1}^{+17.8}$ ppm (2$\sigma$), much lower than seen in interplanetary dust particles and the least altered CR, CO and ungrouped C chondrites, but close to that reported for CM chondrites. NanoSIMS mapping also revealed an unusual $^{13}$C-enriched ($\delta ^{13}$C$\sim$ 100--200 permil) carbonaceous rim surrounding a 1.4 $\mu$m diameter phyllosilicate grain. TEM analysis of two presolar grains with a likely origin in asymptotic giant branch stars identified one as enstatite and one as Al-Mg spinel with minor Cr. The enstatite grain amorphized rapidly under the electron beam, suggesting partial hydration. TEM data of NWA 5958 matrix confirm that it has experienced aqueous alteration and support the suggestion of Jacquet et al. (2016) that this meteorite has affinities to CM2 chondrites.

arXiv:1909.07226v1 [pdf, other]
Image Domain Gridding: a fast method for convolutional resampling of visibilities

In radio astronomy obtaining a high dynamic range in synthesis imaging of wide fields requires a correction for time and direction-dependent effects. Applying direction-dependent correction can be done by either partitioning the image in facets and applying a direction-independent correction per facet, or by including the correction in the gridding kernel (AW-projection). An advantage of AW-projection over faceting is that the effectively applied beam is a sinc interpolation of the sampled beam, where the correction applied in the faceting approach is a discontinuous piece wise constant beam. However, AW-projection quickly becomes prohibitively expensive when the corrections vary over short time scales. This occurs for example when ionospheric effects are included in the correction. The cost of the frequent recomputation of the oversampled convolution kernels then dominates the total cost of gridding. Image domain gridding is a new approach that avoids the costly step of computing oversampled convolution kernels. Instead low-resolution images are made directly for small groups of visibilities which are then transformed and added to the large $uv$ grid. The computations have a simple, highly parallel structure that maps very well onto massively parallel hardware such as graphical processing units (GPUs). Despite being more expensive in pure computation count, the throughput is comparable to classical W-projection. The accuracy is close to classical gridding with a continuous convolution kernel. Compared to gridding methods that use a sampled convolution function, the new method is more accurate. Hence the new method is at least as fast and accurate as classical W-projection, while allowing for the correction for quickly varying direction-dependent effects.

arXiv:1909.07237v1 [pdf, other]
An Introduction to Particle Acceleration in Shearing Flows
Comments: invited review paper; Galaxies, special issue "Particle Acceleration Processes in Astrophysical Jets"; accepted; 21 pages, 14 figures, 95 references

Shear flows are ubiquitously present in space and astrophysical plasmas. This paper highlights the central idea of the non-thermal acceleration of charged particles in shearing flows and reviews some of the recent developments. Topics include the acceleration of charged particles by microscopic instabilities in collisionless relativistic shear flows, Fermi-type particle acceleration in macroscopic, gradual and non-gradual shear flows, as well as shear particle acceleration by large-scale velocity turbulence. When put in the context of jetted astrophysical sources such as Active Galactic Nuclei, the results illustrate a variety of means beyond conventional diffusive shock acceleration by which power-law like particle distributions might be generated. This suggests that relativistic shear flows can account for efficient in-situ acceleration of energetic electrons and be of relevance for the production of extreme cosmic rays.

arXiv:1909.07241v1 [pdf, other]
Late-outburst radio flaring in SS Cyg and evidence for a powerful kinetic output channel in cataclysmic variables
Comments: Accepted for publication as a letter in MNRAS

Accreting white dwarfs in binary systems known as cataclysmic variables (CVs) have in recent years been shown to produce radio flares during outbursts, qualitatively similar to those observed from neutron star and black hole X-ray binaries, but their ubiquity and energetic significance for the accretion flow has remained uncertain. We present new radio observations of the CV SS Cyg with Arcminute Microkelvin Imager Large Array, which show for the second time late-ouburst radio flaring, in April 2016. This flaring occurs during the optical flux decay phase, about ten days after the well-established early-time radio flaring. We infer that both the early- and late-outburst flares are a common feature of the radio outbursts of SS Cyg, albeit of variable amplitudes, and probably of all dwarf novae. We furthermore present new analysis of the physical conditions in the best-sampled late-outburst flare, from Feb 2016, which showed clear optical depth evolution. From this we can infer that the synchrotron-emitting plasma was expanding at about 1% of the speed of light, and at peak had a magnetic field of order 1 Gauss and total energy content > 10^{33} erg. While this result is independent of the geometry of the synchrotron-emitting region, the most likely origin is in a jet carrying away a significant amount of the available accretion power.

arXiv:1909.07246v1 [pdf, other]
He II Emission from Wolf-Rayet Stars as a Tool for Measuring Dust Reddening
Comments: Accepted by The Astronomical Journal

We calibrated a technique to measure dust attenuation in star-forming galaxies. The technique utilizes the stellar-wind lines in Wolf-Rayet stars, which are widely observed in galaxy spectra. The He II 1640 and 4686 features are recombination lines whose ratio is largely determined by atomic physics. Therefore they can serve as a stellar dust probe in the same way as the Balmer lines are used as a nebular probe. We measured the strength of the He II 1640 line in 97 Wolf-Rayet stars in the Galaxy and the Large Magellanic Cloud. The reddening corrected fluxes follow a tight correlation with a fixed ratio of 7.76 for the He II 1640 to 4686 line ratio. Dust attenuation decreases this ratio. We provide a relation between the stellar E(B-V) and the observed line ratio for several attenuation laws. Combining this technique with the use of the nebular Balmer decrement allows the determination of the stellar and nebular dust attenuation in galaxies and can probe its effects at different stellar age and mass regimes, independently of the initial mass function and the star-formation history. We derived the dust reddening from the He II line fluxes and compared it to the reddening from the Balmer decrement and from the slope of the ultraviolet continuum in two star-forming galaxies. The three methods result in dust attenuations which agree to within the errors. Future application of this technique permits studies of the stellar dust attenuation compared to the nebular attenuation in a representative galaxy sample.

arXiv:1909.07255v1 [pdf, other]
Thermal evolution of rocky exoplanets with a graphite outer shell
Comments: Accepted for publication in Astronomy & Astrophysics

The presence of rocky exoplanets with a large refractory carbon inventory is predicted by chemical evolution models of protoplanetary disks of stars with photospheric C/O >0.65, and by models studying the radial transport of refractory organic carbon. High-pressure high-temperature laboratory experiments show that most of the carbon in these exoplanets differentiates into a graphite outer shell. Our aim is to evaluate the effects of a graphite outer shell on the thermal evolution of rocky exoplanets containing a metallic core and a silicate mantle. We implement a parameterized model of mantle convection to determine the thermal evolution of rocky exoplanets with graphite layer thicknesses up to 1000 km. We find that, due to the high thermal conductivity of graphite, conduction is the dominant heat transport mechanism in a graphite layer for the long-term evolution (>200 Myr). The conductive graphite shell essentially behaves like a stagnant lid with a fixed thickness. Models of Kepler-37b (Mercury-size) and a Mars-sized exoplanet show that a planet with a graphite lid cools faster than a planet with a silicate lid, and a planet without a stagnant lid cools the fastest. A graphite lid needs to be approximately ten times thicker than a corresponding silicate lid in order to produce similar thermal evolution.

arXiv:1909.07259v1 [pdf, other]
The mass of dusty clumps with temperature and density structure

We consider a dusty clump in the two cases of spherical and cylindrical symmetry to investigate the effect of temperature and density gradients on the observed flux density. Conversely, we evaluate how the presence of such gradients affects the calculation of the clump mass from the observed flux. We provide the reader with approximate expressions relating flux density and mass in the optically thick and thin limits, in the Rayleigh-Jeans regime, and discuss the reliability of these expressions by comparing them to the outcome of a numerical code. Finally, we present an application of our calculations to three examples taken from the literature, which shows how the correction introduced after taking into account temperature and density gradients may affect our conclusions on the stability of the clumps.

arXiv:1909.07264v1 [pdf, other]
Insights into the physics when modeling cold gas clouds in a hot plasma
Comments: 5 pages, 4 figures, accepted by MNRAS

This paper aims at studying the reliability of a few frequently raised but not proven arguments for the modeling of cold gas clouds embedded in or moving through a hot plasma and at sensitizing modelers to a more careful consideration of unavoidable acting physical processes and their relevance. At first, by numerical simulations we demonstrate the growing effect of self-gravity on interstellar clouds and, by this, moreover argue against their initial setup as homogeneous. We apply the adaptive-mesh refinement code {\sc Flash} with extensions to metal-dependent radiative cooling and external heating of the gas, self-gravity, mass diffusion, and semi-analytic dissociation of molecules and ionization of atoms. We show that the criterion of Jeans mass or Bonnor-Ebert mass, respectively, provides only a sufficient but not a necessary condition for self-gravity to be effective, because even low-mass clouds are affected on reasonable dynamical timescales. The second part of this paper is dedicated to analytically study the reduction of heat conduction by a magnetic dipole field.We demonstrate that in this configuration, the effective heat flow, i.e. integrated over the cloud surface, is suppressed by only $32$ per cent by magnetic fields in energy equipartition and still insignificantly for even higher field strengths.

arXiv:1909.07274v1 [pdf, other]
Efficient wavefront sensing for space-based adaptive optics

Future large space telescopes will be equipped with adaptive optics (AO) to overcome wavefront aberrations and achieve high contrast for imaging faint astronomical objects, such as earth-like exoplanets and debris disks. In contrast to AO that is widely used in ground telescopes, space-based AO systems will use focal plane wavefront sensing to measure the wavefront aberrations. Focal plane wavefront sensing is a class of techniques that reconstruct the light field based on multiple focal plane images distorted by deformable mirror (DM) probing perturbations. In this paper, we report an efficient focal plane wavefront sensing approach for space-based AO which optimizes the DM probing perturbation and thus also the integration time for each image. Simulation of the AO system equipped with a vortex coronagraph has demonstrated that our new approach enables efficient information acquisition and significantly reduces the time needed for achieving high contrast in space.

arXiv:1909.07275v1 [pdf, other]
Globular clusters in the era of precision astrometry
Comments: 4 pages, 2 figure, to appear in Proceedings IAU Symposium No. 351 Star Clusters: From the Milky Way to the Early Universe, 2019

The study of the kinematics of globular clusters (GCs) offers the possibility of unveiling their long term evolution and uncovering their yet unknown formation mechanism. Gaia DR2 has strongly revitalized this field and enabled the exploration of the 6D phase-space properties of Milky Way GCs, thanks to precision astrometry. However, to fully leverage on the power of precision astrometry, a thorough investigations of the data is required. In this contribution, we show that the study of the mean radial proper motion profiles of GCs offers an ideal benchmark to assess the presence of systematics in crowded fields. Our work demonstrates that systematics in Gaia DR2 for the closest 14 GCs are below the random measurement errors, reaching a precision of ~0.015 mas/yr for mean proper motion measurements. Finally, through the analysis of the tangential component of proper motions, we report the detection of internal rotation in a sample of ~50 GCs, and outline the implications of the presence of angular momentum for the formation mechanism of proto-GC. This result gives the first taste of the unparalleled power of Gaia DR2 for GCs science, in preparation for the subsequent data releases.

arXiv:1909.07286v1 [pdf, other]
An Argument for a Kilometer-Scale Nucleus of C/2019 Q4
Comments: 3 pages, 1 figure; submitted for publication

C/2019 Q4 (Borisov), discovered by Gennady Borisov on August 30, 2019, is tentatively an interstellar comet. We show that a kilometer-scale nucleus would provide consistency with both 'Oumuamua and CNEOS-2014-01-08, resulting in a single power-law distribution with an equal amount of mass per logarithmic bin of interstellar objects.

arXiv:1909.07291v1 [pdf, other]
Godbillon-Vey Helicity and Magnetic Helicity in Magnetohydrodynamics
Comments: 42 pages, 10 figures, accepted for publication in Journal of Plasma Physics

The Godbillon-Vey invariant occurs in homology theory, and algebraic topology, when conditions for a co-dimension 1, foliation of a 3D manifold are satisfied. The magnetic Godbillon-Vey helicity invariant in magnetohydrodynamics (MHD) is a higher order helicity invariant that occurs for flows, in which the magnetic helicity density $h_m={\bf A}{\bf\cdot}{\bf B}={\bf A}{\bf\cdot}(\nabla\times{\bf A})=0$, where ${\bf A}$ is the magnetic vector potential and ${\bf B}$ is the magnetic induction. This paper obtains evolution equations for the magnetic Godbillon-Vey field $\boldsymbol{\eta}={\bf A}\times{\bf B}/|{\bf A}|^2$ and the Godbillon-Vey helicity density $h_{gv}=\boldsymbol{\eta}{\bf\cdot}(\nabla\times{\boldsymbol\eta})$ in general MHD flows in which either $h_m=0$ or $h_m\neq 0$. A conservation law for $h_{gv}$ occurs in flows for which $h_m=0$. For $h_m\neq 0$ the evolution equation for $h_{gv}$ contains a source term in which $h_m$ is coupled to $h_{gv}$ via the shear tensor of the background flow. The transport equation for $h_{gv}$ also depends on the electric field potential $\psi$, which is related to the gauge for ${\bf A}$, which takes its simplest form for the advected ${\bf A}$ gauge in which $\psi={\bf A\cdot u}$ where ${\bf u}$ is the fluid velocity. An application of the Godbillon-Vey magnetic helicity to nonlinear force-free magnetic fields used in solar physics is investigated. The possible uses of the Godbillon-Vey helicity in zero helicity flows in ideal fluid mechanics, and in zero helicity Lagrangian kinematics of three-dimensional advection are discussed.

arXiv:1909.07304v1 [pdf, other]
Discovery and Rapid Follow-up Observations of the Unusual Type II SN 2018ivc in NGC 1068

We present the discovery and high-cadence follow-up observations of SN 2018ivc, an unusual Type II supernova that exploded in NGC 1068 (D = 10.1 Mpc). The light curve of SN 2018ivc declines piecewise-linearly, changing slope frequently, with four clear slope changes in the first 40 days of evolution. This rapidly changing light curve indicates that interaction between the circumstellar material and ejecta plays a significant role in the evolution. Circumstellar interaction is further supported by a strong X-ray detection. The spectra are rapidly evolving and dominated by hydrogen, helium, and calcium emission lines. We identify a rare high-velocity emission-line feature blueshifted at ~7800 km/s (in Ha, Hb, Pb, Pg, HeI, CaII), which is visible from day 18 until at least day 78 and could be evidence of an asymmetric progenitor or explosion. From the overall similarity between SN 2018ivc and SN 1996al, the Ha equivalent width of its parent HII region, and constraints from pre-explosion archival Hubble Space Telescope images, we find that SN 2018ivc may have had a high-mass progenitor (with initial mass > 25 Msun). SN 2018ivc demonstrates the importance of the early discovery and rapid follow-up observations of nearby supernovae to study the physics and progenitors of these cosmic explosions.

arXiv:1909.07323v1 [pdf, other]
Surprisingly Strong K-band Emission Found in Low Luminosity Active Galactic Nuclei
Comments: Submitted to ApJ, manuscript after response to referee

We examine the near-infrared (NIR) emission from low-luminosity AGNs (LLAGNs). Our galaxy sample includes 15 objects with detected 2-10 keV X-ray emission, dynamical black hole mass estimates from the literature, and available Gemini/NIFS integral field spectroscopy (IFU) data. We find evidence for red continuum components at the center of most galaxies, consistent with the hot dust emission seen in higher luminosity AGN. We decompose the spectral data cubes into a stellar and continuum component, assuming the continuum component comes from thermal emission from hot dust. We detect nuclear thermal emission in 14 out of 15 objects. This emission causes weaker CO absorption lines and redder continuum ($2.05-2.28\:\mu$m) in our $K$-band data, as expected from hot dust around an AGN. The NIR emission is clearly correlated with the 2-10 keV X-ray flux, with a Spearman coefficient of $r_{spearman}=0.69$ suggesting a $>99\%$ significance of correlation, providing further evidence of an AGN origin. Our sample has typical X-ray and NIR fluxes $3-4$ orders of magnitude less luminous than previous work studying the NIR emission from AGN. We find that the ratio of NIR to X-ray emission increases towards lower Eddington ratios. The NIR emission in our sample is often brighter than the X-ray emission, with our $K$-band AGN luminosities comparable to or greater than the 2-10 keV X-ray luminosities in all objects with Eddington ratios below $0.01\%$. The nature of this LLAGN NIR emission remains unclear, with one possibility being an increased contribution from jet emission at these low luminosities. These observations suggest JWST will be a useful tool for detecting the lowest luminosity AGN.

arXiv:1909.07325v1 [pdf, other]
Fundamental physics behind solar flares

Differential rotation, shear and thermal convection, among other things, produce complex patterns of turbulent flows in magnetized astrophysical systems. The corresponding magnetic fields are consequently entangled in an extremely complicated way. Once the field becomes very entangled, it should slip through the fluid to reduce its spatial complexity level, otherwise the observed large scale fields in astrophysical objects could never be generated and evolved over cosmological time scales. Such a spontaneous slippage of magnetic fields launching jets of fluid-magnetic reconnection-is usually interpreted and described as a change in the topology of the stochastic magnetic fields. However, neither magnetic topology nor its stochasticity level is usually given a precise mathematical definition, and such technical terms are usually used rather loosely. We show that in fact magnetic topology is well-defined only in the phase space corresponding to a dynamical system governed by the induction equation. Hence the field's topology and stochasticity should be studied in terms of the corresponding phase space trajectories rather than the field lines in real Euclidean space. It is shown that the phase space topology is preserved in time for a magnetic field which, besides satisfying few continuity conditions, solves a time reversal invariant induction equation. What breaks the time symmetry in the induction equation is the presence of non-ideal plasma effects at small scales such as resistivity, which results from random collisions between diffusing electrons and other particles. This suggests that reconnection is rooted in the second law of thermodynamics that dictates entropy increase which in turn breaks the time symmetry.

arXiv:1909.07338v1 [pdf, other]
An evolving broad iron line from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124
Comments: 11 pages, 8 Figures, Accepted for publication in ApJ

We present a spectral study of the ultraluminous Be/X-ray transient pulsar Swift J0243.6+6124 using Neutron Star Interior Composition Explorer (NICER) observations during the system's 2017--2018 giant outburst. The 1.2--10~keV energy spectrum of the source can be approximated with an absorbed cut-off power law model. We detect strong, luminosity-dependent emission lines in the 6--7 keV energy range. A narrow 6.42 keV line, observed in the sub-Eddington regime, is seen to evolve into a broad Fe-line profile in the super-Eddington regime. Other features are found at 6.67 and 6.97 keV in the Fe-line complex. An asymmetric broad line profile, peaking at 6.67 keV, is possibly due to Doppler effects and gravitational redshift. The 1.2--79 keV broadband spectrum from NuSTAR and NICER observations at the outburst peak is well described by an absorbed cut-off power law plus multiple Gaussian lines and a blackbody component. Physical reflection models are also tested to probe the broad iron line feature. Depending on the mass accretion rate, we found emission sites that are evolving from ~5000 km to a range closer to the surface of the neutron star. Our findings are discussed in the framework of the accretion disk and its implication on the magnetic field, the presence of optically thick accretion curtain in the magnetosphere, jet emission, and the massive, ultra-fast outflow expected at super-Eddington accretion rates. We do not detect any signatures of a cyclotron absorption line in the NICER or NuSTAR data.

arXiv:1909.07340v1 [pdf, other]
Generating Synthetic Cosmological Data with GalSampler

As part of the effort to meet the needs of the Large Synoptic Survey Telescope Dark Energy Science Collaboration (LSST DESC) for accurate, realistically complex mock galaxy catalogs, we have developed GalSampler, an open-source python package that assists in generating large volumes of synthetic cosmological data. The key idea behind GalSampler is to recast hydrodynamical simulations and semi-analytic models as physically-motivated galaxy libraries. GalSampler populates a new, larger-volume halo catalog with galaxies drawn from the baseline library; by using weighted sampling guided by empirical modeling techniques, GalSampler inherits statistical accuracy from the empirical model and physically-motivated complexity from the baseline library. We have recently used GalSampler to produce the cosmoDC2 extragalactic catalog made for the LSST DESC Data Challenge 2. Using cosmoDC2 as a guiding example, we outline how GalSampler can continue to support ongoing and near-future galaxy surveys such as the Dark Energy Survey (DES), the Dark Energy Spectroscopic Instrument (DESI), WFIRST, and Euclid.

arXiv:1909.07346v1 [pdf, other]
Deep Learning the Morphology of Dark Matter Substructure

Strong gravitational lensing is a promising probe of the substructure of dark matter halos. Deep learning methods have the potential to accurately identify images containing substructure, and differentiate WIMP dark matter from other well motivated models, including vortex substructure of dark matter condensates and superfluids. This is crucial in future efforts to identify the true nature of dark matter. We implement, for the first time, a classification approach to identifying dark matter substructure based on simulated strong lensing images with different substructure. Utilizing convolutional neural networks trained on sets of simulated images, we demonstrate the feasibility of deep neural networks to reliably distinguish among different types of dark matter substructure. With thousands of strong lensing images anticipated with the coming launch of LSST, we expect that supervised and unsupervised deep learning models will play a crucial role in determining the nature of dark matter.

arXiv:1909.07356v1 [pdf, other]
Spectral diagnostics of cool flare loops observed by SST: I. Inversion of the Ca II 8542 Å and H$β$ lines
Comments: 16 pages, 9 figures; accepted for publication in The Astrophysical Journal on September 11, 2019

Flare loops form an integral part of eruptive events, being detected in the range of temperatures from X-rays down to cool chromospheric-like plasmas. While the hot loops are routinely observed by the Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA), cool loops seen off-limb are rare. In this paper we employ unique observations of the SOL2017-09-10T16:06 X8.2-class flare which produced an extended arcade of loops. The Swedish 1-m Solar Telescope (SST) made a series of spectral images of the cool off-limb loops in the Ca II 8542 \r{A} and the hydrogen H$\beta$ lines. Our focus is on the loop apices. Non-LTE spectral inversion is achieved through the construction of extended grids of models covering a realistic range of plasma parameters. The Multilevel Accelerated Lambda Iterations (MALI) code solves the non-LTE radiative-transfer problem in a 1D externally-illuminated slab, approximating the studied loop segment. Inversion of the Ca II 8542 \r{A} and H$\beta$ lines yields two similar solutions, both indicating high electron densities around $2 \times 10^{12}$ cm$^{-3}$ and relatively large microturbulence around 25 kms$^{-1}$. These are in reasonable agreement with other independent studies of the same or similar events. In particular, the high electron densities in the range $10^{12} - 10^{13}$ cm$^{-3}$ are consistent with those derived from the SDO's Helioseismic and Magnetic Imager white-light observations. The presence of such high densities in solar eruptive flares supports the loop interpretation of the optical continuum emission of stars which manifest superflares.

arXiv:1909.07366v1 [pdf, other]
Consistency relations for large-scale structure in modified gravity and the matter bispectrum