### 40 articles on Sunday, October 21

arXiv:1810.08203v1 [pdf, other]
The Intrinsic Stochasticity of the $^{56}$Ni Distribution of Single-Degenerate Type Ia Supernovae

Binary Chandrasekhar-mass white dwarfs accreting mass from non-degenerate stellar companions through the single-degenerate channel have reigned for decades as the leading explanation of Type Ia supernovae. Yet, a comprehensive theoretical explanation has not yet emerged to explain the expected properties of the canonical near-Chandrasekhar-mass white dwarf model. A simmering phase within the convective core of the white dwarf leads to the ignition of one or more flame bubbles scattered across the core. Consequently, near-Chandrasekhar-mass single-degenerate SNe Ia are inherently stochastic, and are expected to lead to a range of outcomes, from subluminous SN 2002cx-like events, to overluminous SN 1991T-like events. However, all prior simulations of the single-degenerate channel carried through the detonation phase have set the ignition points as free parameters. In this work, for the first time, we place ignition points as predicted by {\it ab initio} models of the convective phase leading up to ignition, and follow through the detonation phase in fully three-dimensional simulations. Single-degenerates in this framework are characteristically overluminous. Using a statistical approach, we determine the $^{56}$Ni mass distribution arising from stochastic ignition. While there is a total spread of $\gtrsim 0.2 M_{\odot}$ for detonating models, the distribution is strongly left-skewed, and with a narrow standard deviation of $\simeq 0.03 M_{\odot}$. Conversely, if single-degenerates are not overluminous but primarily yield normal or failed events, then the models require fine-tuning of the ignition parameters, or otherwise require revised physics or progenitor models. We discuss implications of our findings for the modeling of single-degenerate SNe Ia.

arXiv:1810.08205v1 [pdf, other]
Magnetohydrodynamic turbulence and turbulent dynamo in a partially ionized plasma
Comments: 25 pages, 9 figures, published in New Journal of Physics

Astrophysical fluids are turbulent, magnetized and frequently partially ionized. As an example of astrophysical turbulence, the interstellar turbulence extends over a remarkably large range of spatial scales and participates in key astrophysical processes happening in different ranges of scales. A significant progress has been achieved in the understanding of the magnetohydrodynamic (MHD) turbulence since the turn of the century, and this enables us to better describe turbulence in magnetized and partially ionized plasmas. In fact, the modern revolutionized picture of the MHD turbulence physics facilitates the development of various theoretical domains, including the damping process for dissipating MHD turbulence and the dynamo process for generating MHD turbulence with many important astrophysical implications. In this paper, we review some important findings from our recent theoretical works to demonstrate the interconnection between the properties of MHD turbulence and those of turbulent dynamo in a partially ionized gas. We also briefly exemplify some new tentative studies on how the revised basic processes influence the associated outstanding astrophysical problems in, such as, magnetic reconnection, cosmic ray scattering, magnetic field amplification in both the early and the present-day universe.

arXiv:1810.08206v1 [pdf, other]
Constraining Black Hole Formation with 2M05215658+4359220

We show that the recently discovered binary 2M05215658+4359220, comprised of a giant star (GS) orbiting a suspected black hole (BH) in a ~80 day orbit, may be instrumental in shedding light on uncertain BH-formation physics and can be a test case for studying wind accretion models. Using binary population synthesis and a realistic prescription of the star formation history and metallicity evolution of the Milky Way, we analyze the formation histories of detached BH-GS binaries like 2M05215658+4359220, and find that all such systems with orbital periods less than 5 years went through a common envelope. Furthermore, the 'rapid' and 'delayed' supernova engine models produce very different BH mass distributions in BH-GS binaries, and one current mass estimate for the BH in 2M05215658+4359220 is inconsistent with the rapid model. An improved measurement of the orbit of 2M05215658+4359220, which we argue is imminent with the next Gaia data release, therefore has widespread implications including for SN engine models and for the types of binaries detectable by LIGO and LISA. Finally, we show that the reported X-ray non-detection is a challenge for wind accretion theory, making 2M05215658+4359220 a prime target for further study with accretion models.

arXiv:1810.08210v1 [pdf, other]
The early impact of ionizing radiation on forming molecular clouds

As part of the SILCC-ZOOM project we present our first sub-parsec resolution radiation-hydrodynamic simulations of two molecular clouds self-consistently forming from a turbulent, multi-phase ISM. The clouds have similar initial masses of few 10$^4$ M$_{\odot}$, escape velocities of ~5 km s$^{-1}$, and a similar initial energy budget. We follow the formation of star clusters with a sink based model and the impact of radiation from individual massive stars with the tree-based radiation transfer module TreeRay. Photo-ionizing radiation is coupled to a chemical network to follow gas heating, cooling and molecule formation and dissociation. For the first 3 Myr of cloud evolution we find that the overall star formation effciency is considerably reduced by a factor of ~4 to global cloud values of < 10 % as the mass accretion of sinks that host massive stars is terminated after <1 Myr. Despite the low effciency, star formation is triggered across the clouds. Therefore, a much larger region of the cloud is affected by radiation and the clouds begin to disperse. The time scale on which the clouds are dispersed sensitively depends on the cloud substructure and in particular on the amount of gas at high visual extinction. The damage of radiation done to the highly shielded cloud (MC1) is delayed. We also show that the radiation input can sustain the thermal and kinetic energy of the clouds at a constant level. Our results strongly support the importance of ionizing radiation from massive stars for explaining the low observed star formation effciency of molecular clouds

arXiv:1810.08211v1 [pdf, other]
Machine Learning Applied to the Reionization History of the Universe
Comments: 10 pages, 5 figures, submitted to ApJ

The Epoch of Reionization (EoR) features a rich interplay between the first luminous sources and the low-density gas of the intergalactic medium (IGM), where photons from these sources ionize the IGM. There are currently few observational constraints on key observables related to the EoR, such as the midpoint and duration of reionization. Although upcoming observations of the 21 cm power spectrum with next-generation radio interferometers such as the Hydrogen Epoch of Reionization Array (HERA) and the Square Kilometre Array (SKA) are expected to provide information about the midpoint of reionization readily, extracting the duration from the power spectrum alone is a more difficult proposition. As an alternative method for extracting information about reionization, we present an application of convolutional neural networks (CNNs) to images of reionization. These images are two-dimensional in the plane of the sky, and extracted at a series of redshift values to generate "image cubes" that are qualitatively similar to those HERA and the SKA will generate in the near future. Additionally, we include the impact that the bright foreground signal from the Milky Way galaxy imparts on such image cubes from interferometers, and degrade the simulated images accordingly. We show that we are able to recover the duration of reionization $\Delta z$ to within 10% using CNNs, assuming that the midpoint of reionization is already relatively well constrained. These results have exciting impacts for estimating $\tau$, the optical depth to the CMB, which can help constrain other cosmological parameters.

arXiv:1810.08212v1 [pdf, other]
Dependency of halo concentration on mass, redshift and fossilness in Magneticum hydrodynamic simulations

We study the dependency of the concentration on mass and redshift using three large N-body cosmological hydrodynamic simulations carried out by the Magneticum project. We find a negative trend on the mass-concentration plane and a slightly negative redshift dependency, in agreement with observations and other numerical works. We constrain the slope of the mass-concentration relation with an unprecedented mass range for hydrodynamic simulations. We also investigate the origin of the large scatter of concentration by studying the relation with the fossil parameter, defined as the stellar mass ratio between the central galaxy and the most massive satellite. We study the correlation between concentration and fossilness by following the evolution of haloes that undergo major merging events and in objects without activity. We find that the internal region keeps accreating satellites and this causes both an increase of the fossil parameter and a slow but steady decrease of the scale radius, which increases the concentration. Finally, we study the dependency of the concentration on the virial ratio and the energy term from the surface pressure $E_s$. We find that the relation between concentration, fossilness and $E_s$ is due to the relation between $E_s$ and the presence of in-falling/out-falling material.

arXiv:1810.08213v1 [pdf, other]
Carnegie Supernova Project-II: The Near-infrared Spectroscopy Program
Comments: 20 pages, 7 figures, accepted for publication in PASP

Shifting the focus of Type Ia supernova (SN Ia) cosmology to the near-infrared (NIR) is a promising way to significantly reduce the systematic errors, as the strategy minimizes our reliance on the empirical width-luminosity relation and uncertain dust laws. Observations in the NIR are also crucial for our understanding of the origins and evolution of these events, further improving their cosmological utility. Any future experiments in the rest-frame NIR will require knowledge of the SN Ia NIR spectroscopic diversity, which is currently based on a small sample of observed spectra. Along with the accompanying paper, Phillips et al. (2018), we introduce the Carnegie Supernova Project-II (CSP-II), to follow up nearby SNe Ia in both the optical and the NIR. In particular, this paper focuses on the CSP-II NIR spectroscopy program, describing the survey strategy, instrumental setups, data reduction, sample characteristics, and future analyses on the data set. In collaboration with the Harvard-Smithsonian Center for Astrophysics (CfA) Supernova Group, we obtained 661 NIR spectra of 157 SNe Ia. Within this sample, 451 NIR spectra of 90 SNe Ia have corresponding CSP-II follow-up light curves. Such a sample will allow detailed studies of the NIR spectroscopic properties of SNe Ia, providing a different perspective on the properties of the unburned material, radioactive and stable nickel produced, progenitor magnetic fields, and searches for possible signatures of companion stars.

arXiv:1810.08214v1 [pdf, other]
Nonlinear Evolution of Instabilities Between Dust and Sound Waves

We study the non-linear evolution of the acoustic 'Resonant Drag Instability' (RDI) using numerical simulations. The acoustic RDI is excited in a dust-gas mixture when dust grains stream through gas, interacting with sound waves to cause a linear instability. We study this process in a periodic box by accelerating neutral dust with an external driving force. The instability grows as predicted by linear theory, eventually breaking into turbulence and saturating. As in linear theory, the non-linear behavior is characterized by three regimes - high, intermediate, and low wavenumbers - the boundary between which is determined by the dust-gas coupling strength and the dust-to-gas mass ratio. The high and intermediate wavenumber regimes behave similarly to one another, with large dust-to-gas ratio fluctuations while the gas remains largely incompressible. The saturated state is highly anisotropic: dust is concentrated in filaments, jets, or plumes along the direction of acceleration, with turbulent vortex-like structures rapidly forming and dissipating in the perpendicular directions. The low-wavenumber regime exhibits large fluctuations in gas and dust density, but the dust and gas remain more strongly coupled in coherent 'fronts' perpendicular to the acceleration. These behaviors are qualitatively different from those of dust 'passively' driven by external hydrodynamic turbulence, with no back-reaction force from dust onto gas. The virulent nature of these instabilities has interesting implications for dust-driven winds in a variety of astrophysical systems, including around cool-stars, in dusty torii around active-galactic-nuclei, and in and around giant molecular clouds.

arXiv:1810.08226v1 [pdf, other]
The physical and chemical properties of the $ρ$ ophiuchi A dense core
Comments: 13 pages, 12 figures, accepted for publication in ApJ

The physical and chemical properties of the $\rho$ Ophiuchi A core were studied using 1.3 mm continuum and molecular lines such as C$^{18}$O, C$^{17}$O, CH$_3$OH and H$_2$CO observed with the Submillimeter Array (SMA). The continuum and C$^{18}$O data were combined with the single-dish data obtained with the IRAM 30m telescope and the James Clerk Maxwell Telescope (JCMT), respectively. The combined 1.3 mm continuum map reveals three major sources, SM1, SM1N and VLA1623 embedded in the extended emission running along the north-south direction, and two additional compact condensations in the continuum ridge connecting SM1 and VLA1623.The spatial distribution of the C$^{18}$O emission is significantly different from that of the continuum emission; the C$^{18}$O emission is enhanced at the eastern and western edges of the continuum ridge, with its peak brightness temperature of 40--50 K. This supports the picture that the $\rho$-Oph A core is heated externally from the nearby stars Oph S1 and HD147889. In contrast, the C$^{18}$O intensity is lower than 15--20 K at the center of the ridge where the continuum emission is bright. The C$^{18}$O abundance decreases inside the ridge, and shows anti-correlation with the N$_2$H$^+$ abundance. However, both C$^{18}$O and N$_2$H$^+$ show strong depletion at the Class 0 protostar VLA1623, implying that the dense gas surrounding VLA1623 is colder than the freeze-out temperature of N$_2$. The blue- and red-shifted components of CH$_3$OH and H$_2$CO lines are seen at SM1, suggesting outflow activity of embedded source in SM1, although the spatial distributions do not show clear bipolarity.

arXiv:1810.08233v1 [pdf, other]
HAWC+/SOFIA Multiwavelength Polarimetric Observations of OMC-1
Comments: 26 pages, 11 figures, submitted to ApJ

We report new polarimetric and photometric maps of the massive star-forming region OMC-1 using the HAWC+ instrument on the Stratospheric Observatory for Infrared Astronomy (SOFIA). We present continuum polarimetric and photometric measurements of this region at 53, 89, 154, and 214 microns at angular resolutions of 5.1, 7.9, 14.0, and 18.7 arcseconds for the four bands, respectively. The photometric maps enable the computation of improved SEDs for the region. We find that at the longer wavelengths, the inferred magnetic field configuration matches the 'hourglass' configuration seen in previous studies, indicating magnetically-regulated star formation. The field morphology differs at the shorter wavelengths. The magnetic field inferred at these wavelengths traces the bipolar structure of the explosive Becklin-Neugebauer (BN)/Kleinman-Low (KL) outflow emerging from OMC-1 behind the Orion Nebula. Using statistical methods to estimate the field strength in the region, we find that the explosion dominates the magnetic field near the center of the feature. Farther out, the magnetic field is close to energetic equilibrium with the ejecta and may be providing confinement to the explosion. The correlation between polarization fraction and the local polarization angle dispersion indicates that the depolarization as a function of unpolarized intensity is a result of intrinsic field geometry as opposed to decreases in grain alignment efficiency in denser regions.

arXiv:1810.08234v1 [pdf, other]
The large-scale structure of the halo of the Andromeda galaxy II. Hierarchical structure in the Pan-Andromeda Archaeological Survey
Comments: Accepted for publication in the Astrophysical Journal. 51 pages, 24 figures, 5 tables. Some figures have degraded resolution. All PAndAS data products are available via the CADC at http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/community/pandas/query.html where you can also find a version of the paper with full resolution figures

The Pan-Andromeda Archaeological Survey is a survey of $>400$ square degrees centered on the Andromeda (M31) and Triangulum (M33) galaxies that has provided the most extensive panorama of a $L_\star$ galaxy group to large projected galactocentric radii. Here, we collate and summarise the current status of our knowledge of the substructures in the stellar halo of M31, and discuss connections between these features. We estimate that the 13 most distinctive substructures were produced by at least 5 different accretion events, all in the last 3 or 4 Gyrs. We suggest that a few of the substructures furthest from M31 may be shells from a single accretion event. We calculate the luminosities of some prominent substructures for which previous estimates were not available, and we estimate the stellar mass budget of the outer halo of M31. We revisit the problem of quantifying the properties of a highly structured dataset; specifically, we use the OPTICS clustering algorithm to quantify the hierarchical structure of M31's stellar halo, and identify three new faint structures. M31's halo, in projection, appears to be dominated by two 'mega-structures', that can be considered as the two most significant branches of a merger tree produced by breaking M31's stellar halo into smaller and smaller structures based on the stellar spatial clustering. We conclude that OPTICS is a powerful algorithm that could be used in any astronomical application involving the hierarchical clustering of points. The publication of this article coincides with the public release of all PAndAS data products.

arXiv:1810.08247v1 [pdf, other]
Anomalous Outbursts of H 1743-322

Using black body and power-law photon counts of All Sky Monitor (ASM) in Rossi X-ray Timing Explorer (RXTE) satellite it has been established recently by us that there is a significant time lag between the infall timescales of two components in the Two-Component Advective Flow (TCAF) paradigm, where a standard slow moving Keplerian disc is surrounded by a fast moving halo. The time lag is clearly due to the difference in viscosity in the flow components and the size of the Keplerian disc may be considered to be proportional to the arrival time lag. In this paper, using RXTE/ASM (1.5-12 keV) data, we examine eight successive outbursts of the low-mass X-ray binary H 1743-322 since 2003 from a new angle. A dynamic photon index, {\Theta} indicates that the size of the Keplerian disc is biggest during the brightest outburst of 2003. The size diminishes thereafter during subsequent weaker outbursts. These results are corroborated when two energy fluxes corresponding to the two flows are cross-correlated with reference to {\Theta}. Moreover, {\Theta} decides spectral transitions of any outburst. We show from the behaviour of {\Theta} alone that the outburst of October 2008 was an anomalous outburst. In fact, each normal outburst was either preceded or followed by an otherwise premature outburst. This makes H 1743-322 an enigmatic source and a subject of further study.

arXiv:1810.08249v1 [pdf, other]
Does Cyg X-1 have a small Standard Accretion Disc?

We analyze several outbursts of a few transient sources using Proportional Counter Array (PCA) data (2.5-25 keV) as well as All Sky Monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) satellite. We find a time delay between the arrival times of the Keplerian disc component and the halo of the Two-Component Advective Flow (TCAF) when the spectral data is fitted with TCAF solution. We compare this time delay from the spectral fits with the TCAF solution of the transient low mass X-ray binaries (LMXBs) e.g., GX 339-4, H 1743-322 and MAXI J1836-194 with those of the high mass X-ray Binaries (HMXBs) such as Cyg X-1. We find that several days of time delays are observed in LMXBs while for Cyg X-1 the delay is negligible. We interpret the large delay to be due to the viscous delay of a large Keplerian component to reach the inner region as compared to nearly free-fall time taken by the low angular momentum halo component. The delay is of the order of a few days for the low mass X-ray binaries (LMXB) where the feeding is primarily through the Roche-lobe. However, it is negligible in a wind-fed system since a very small Keplerian disc is created here by slowly redistributing the low angular momentum of the wind.

arXiv:1810.08250v1 [pdf, other]
Signature of Two-Component Advective Flow in several Black Hole candidates obtained through time-of-arrival analysis of RXTE/ASM Data

We study several Galactic black hole candidates using long-time RXTE/ASM X-ray data to search for telltale signatures of differences in viscous timescales in the two components used in the Two-Component Advective Flow (TCAF) paradigm. In high-mass X-ray binaries (HMXBs) mainly winds are accreted. This nearly inviscid and dominant sub-Keplerian flow falls almost freely towards the black hole. A standard Keplerian disc can form out of this sub-Keplerian matter in presence of a significant viscosity and could be small in size. However, in low-mass X-ray binaries (LMXBs), highly viscous and larger Keplerian accretion disc is expected to form inside the sub-Keplerian disc due to the Roche-lobe overflow. Due to two viscous timescales in these two components, it is expected to have a larger lag between the times-of-arrival of these components in LMXBs than that in HMXBs. Direct cross-correlation between the photon fluxes will not reveal this lag/delay since they lack linear dependence; however, they are coupled through the viscous processes which bring in both matter. To quantify the aforesaid time lag, we introduce an index ({\Theta}), which is a proxy of the usual photon index ({\Gamma}). Thus, when {\Theta}, being dynamically responsive to both fluxes, is considered as a reference, the arrival time lag between the two fluxes in LMXBs is found to be much larger than that in HMXBs. Our result establishes the presence of two dynamical components in accretion and shows that the Keplerian disc size indeed is smaller in HMXBs as compared to that in LMXBs.

arXiv:1810.08258v1 [pdf, other]
Science with an ngVLA: Imaging Cold Gas to 1 kpc Scales in High-Redshift Galaxies with the ngVLA
Comments: To be published in the ASP Monograph Series, "Science with a Next-Generation VLA", ed. E. J. Murphy (ASP, San Francisco, CA)

The next generation Very Large Array (ngVLA) will revolutionize our understanding of the distant Universe via the detection of cold molecular gas in the first galaxies. Its impact on studies of galaxy characterization via detailed gas dynamics will provide crucial insight on dominant physical drivers for star-formation in high redshift galaxies, including the exchange of gas from scales of the circumgalactic medium down to resolved clouds on mass scales of $\sim10^{5}\,M_\odot$. In this study, we employ a series of high-resolution, cosmological, hydrodynamic zoom simulations from the MUFASA simulation suite and a CASA simulator to generate mock ngVLA observations of a $z\sim4.5$ gas rich star-forming galaxy. Using the DESPOTIC radiative transfer code that encompasses simultaneous thermal, chemical, and statistical equilibrium in calculating the molecular and atomic level transitions of CO from ALMA for comparison. We find that observations of CO(1-0) are especially important for tracing the systemic redshift of the galaxy and the total mass of the well-shielded molecular gas reservoir, while even CO(2-1) can predominantly trace denser gas regions distinct from CO(1-0). The factor of 100 times improvement in mapping speed for the ngVLA beyond the Jansky VLA and the proposed ALMA Band 1 will make these detailed, high-resolution imaging and kinematic studies of CO(1-0) routine at $z\sim2-5$.

arXiv:1810.08261v1 [pdf, other]
Simulating the Collapse of a Thick Accretion Disk due to a Type I X-ray Burst from a Neutron Star
Comments: 7 pages, 3 figures, to appear in ApJ Letters

We use two-dimensional, general relativistic, viscous, radiation hydrodynamic simulations to study the impact of a Type I X-ray burst on a hot and geometrically thick accretion disk surrounding an unmagnetized, non-rotating neutron star. The disk is initially consistent with a system in its low/hard spectral state, and is subject to a burst which rises to a peak luminosity of $10^{38}$ erg s$^{-1}$ in $2.05$ s. At the peak of the burst, the temperature of the disk has dropped by more than three orders of magnitude and its scale height has gone down by more than one order of magnitude. The simulations show that these effects predominantly happen due to Compton cooling of the hot plasma, and clearly illustrate the potential cooling effects of bursts on accretion disk coronae. In addition, we demonstrate the presence of Poynting-Robertson drag, though it only enhances the mass accretion rate onto the neutron star by a factor of $\sim 3$-$4$ compared to a simulation with no burst. Simulations such as these are important for building a general understanding of the response of an accretion disk to an intense X-ray impulse, which, in turn, will be crucial for deciphering burst spectra. Detailed analysis of such spectra offers the potential to measure neutron star radii, and hence constrain the neutron star equation of state, but only if the contributions coming from the impacted disk and its associated corona can be understood.

arXiv:1810.08269v1 [pdf, other]
The NANOGrav 11-year Data Set: Pulse Profile Variability

Access to 50 years of data has led to the discovery of pulsar emission and rotation variability on timescales of months and years. Most of this long-term variability has been seen in long-period pulsars, with relatively little focus on recycled millisecond pulsars. We have analyzed a 38-pulsar sub-set of the 45 millisecond pulsars in the NANOGrav 11-year data set, in order to review their pulse profile stability. The most variability, on any timescale, is seen in PSRs J1713+0747, B1937+21 and J2145-0750. The strongest evidence for long-timescale pulse profile changes is seen in PSRs B1937+21 and J1643-1224. We have focused our analyses on these four pulsars in an attempt to elucidate the causes of their profile variability. Effects of scintillation seem to be responsible for the profile modifications of PSR J2145-0750. We see evidence that imperfect polarization calibration contributes to the profile variability of PSRs J1713+0747 and B1937+21, along with radio frequency interference around 2 GHz, but find that propagation effects also have an influence. The changes seen in PSR J1643-1224 have been reported previously, yet elude explanation beyond their astrophysical nature. Regardless of cause, unmodeled pulse profile changes are detrimental to the accuracy of pulsar timing and must be incorporated into the timing models where possible.

arXiv:1810.08274v1 [pdf, other]
3C 17: The BCG of a galaxy cluster at z=0.22

Gemini Multi Object Spectrograph medium-resolution spectra and photometric data of 39 objects in the field of the radio galaxy 3C 17 are presented. Based on the new data, a previously uncataloged cluster of galaxies is identified at a mean redshift of z=0.220 +/-0.003, a projected virial radius of 0.37 Mpc, and a velocity dispersion of sigma_v = 821 +/-171 km/s. The brightest member of this cluster is 3C 17 with M_r = -22.45 mag. The surface brightness profile of 3C 17 is best fit with two components (Exponential + Sersic) characteristic of brightest cluster galaxies. The spectrum of 3C 17 is dominated by broad emission lines Halpha + N[II] and Hbeta + [OIII]. Analysis of Chandra data shows extended emission around the cluster core that supports the existence of hot gas cospatial with 3C 17. The discovery of a cluster of galaxies around 3C 17 better explains the sharply bent morphology of the radio jet given that it propagates through a dense intracluster medium.

arXiv:1810.08301v1 [pdf, other]
SOFIA FIFI-LS Observations of Sgr B1: Ionization Structure and Sources of Excitation
Comments: 9 pages, 3 figures. Accepted by ApJL

The current paradigm of Galactic Center (GC) gas motions and star formation envisions sequential star formation in streams of gas as they pass near the supermassive black hole, Sgr A*. This is based on the relative positions of dense molecular clouds, the very young star-forming region Sgr B2, the much older region Sgr C, and the several Myr old Arches and Quintuplet Clusters. Because Sgr B1 is found with Sgr B2 in a common envelope of molecular gas and far-infrared emission, the two sources are thought to be physically related, even though there are indicators of a significantly greater age for Sgr B1. To clarify the status of Sgr B1, we have mapped it with the FIFI-LS spectrometer on SOFIA in the far-infrared lines of [O III] 52 and 88 micron. From the ratios of these lines and lines measured with the Spitzer Infrared Spectrograph, we find that there are at least eight separate sub-regions that must contain the stars that excite the gas. We infer spectral energy distributions (SEDs) of the ionizing sources from models and find they are in agreement only with SEDs of late O stars augmented at the highest frequencies with interstellar X-rays from fast shocks. We suggest that although the gas, from its velocity structure, must be part of the very young Sgr B2 complex, the stars that are ionizing the gas were not formed there but are the remnants of a previous generation of star formation in the GC.

arXiv:1810.08340v1 [pdf, other]
The Morphology of Hα emission in CALIFA galaxies
Comments: 16 pages, 12 figures, submitted to MNRAS

We have determined the H{\alpha} emission line radial profiles for a sample of 86 face-on galaxies observed in the CALIFA survey and analyzed with the Pipe3D pipeline. From a visual analysis we propose a two step classification of these profiles. Initially, they were divided in two classes with respect to the maximum of the H{\alpha} emission: C (for central) profiles have the maximum line emission at the galaxy center, whereas EX (for extended) profiles have the maximum H{\alpha} emission outside the galaxy center. After, we divided the C galaxies in two classes, CE and CL (where E and L stands here for 'early' and 'late'), through the value of $c_r$, the concentration index in the r-band. We analyzed the profile class dependence of several galaxy parameters, as well as examined the nature of line emission through the BPT diagram. We notice that almost 75% of the sample is in the C class. Elliptical and S0 galaxies dominate the CE class, with spiral galaxies being found mostly in the CL and EX classes. We also notice that spirals in each of these classes have different properties, with CL objects seeming less evolved than those in the EX class.

arXiv:1810.08423v1 [pdf, other]
Classification of Subpulse Drifting in Pulsars
Comments: 13 pages, 3 figures, additional 36 figures in appendix, accepted for publication in MNRAS

In this study we propose a classification scheme for the phenomenon of subpulse drifting in pulsars. We have assembled an exhaustive list of pulsars which exhibit subpulse drifting from previously published results as well as recent observations using the Giant Meterwave Radio Telescope. We have estimated detailed phase variations corresponding to the drifting features. Based on phase behaviour the drifting population was classified into four groups : coherent phase-modulated drifting, switching phase-modulated drifting, diffuse phase-modulated drifting and low-mixed phase-modulated drifting. We have re-established the previous assertion that the subpulse drifting is primarily associated with the conal components in pulsar profile. The core components generally do not show the drifting phenomenon. However, in core emission of certain pulsars longer periodic fluctuations are seen, which are similar to periodic nulling, and likely arise due to a different physical phenomenon. In general the nature of the phase variations of the drifting features across the pulsar profile appears to be associated with specific pulsar profile classes, but we also find several examples that show departures from this trend. It has also been claimed in previous works that the spin-down energy loss is anti-correlated with the drifting periodicity. We have verified this dependence using a larger sample of drifting measurements.

arXiv:1810.08430v1 [pdf, other]
An application of machine learning techniques to galaxy cluster mass estimation using the MACSIS simulations
Comments: Submitted to MNRAS, 12 pages, 7 figures, example pipeline can be found here: https://github.com/TomArmitage/ML_Template

Machine learning (ML) techniques, in particular supervised regression algorithms, are a promising new way to use multiple observables to predict a cluster's mass or other key features. To investigate this approach we use the \textsc{MACSIS} sample of simulated hydrodynamical galaxy clusters to train a variety of ML models, mimicking different datasets. We find that compared to predicting the cluster mass from the $\sigma -M$ relation, the scatter in the predicted-to-true mass ratio is reduced by a factor of 4, from $0.130\pm0.004$ dex (${\simeq} 35$ per cent) to $0.031 \pm 0.001$ dex (${\simeq} 7$ per cent) when using the same, interloper contaminated, spectroscopic galaxy sample. Interestingly, omitting line-of-sight galaxy velocities from the training set has no effect on the scatter when the galaxies are taken from within $r_{200c}$. We also train ML models to reproduce estimated masses derived from mock X-ray and weak lensing analyses. While the weak lensing masses can be recovered with a similar scatter to that when training on the true mass, the hydrostatic mass suffers from significantly higher scatter of ${\simeq} 0.13$ dex (${\simeq} 35$ per cent). Training models using dark matter only simulations does not significantly increase the scatter in predicted cluster mass compared to training on simulated clusters with hydrodynamics. In summary, we find ML techniques to offer a powerful method to predict masses for large samples of clusters, a vital requirement for cosmological analysis with future surveys.

arXiv:1810.08441v1 [pdf, other]
STiC -- A multi-atom non-LTE PRD inversion code for full-Stokes solar observations
Comments: Submitted to Astronomy & Astrophysics

The inference of the underlying state of the plasma in the solar chromosphere remains extremely challenging because of the non-local character of the observed radiation and of the plasma conditions in this layer. Inversion methods allow deriving a model atmosphere that can reproduce the observed spectra by undertaking several physical assumptions. The most advanced variant of these codes work with a depth-stratified model atmosphere including temperature, line-of-sight velocity, turbulent velocity, the three components of the magnetic field vector, and gas and electron pressure. The parameters of the radiative transfer equation are computed from a solid ground of physical principles. In order to apply these techniques to spectral lines that sample the chromosphere, non-local thermodynamical equilibrium effects must be included in the calculations. We have developed a new inversion code (STiC) to study spectral lines that sample the upper chromosphere. The code is based on the RH forward synthesis code that we have modified to make the inversions faster and more stable. For the first time, STiC allows processing lines from multiple atoms in non-LTE, also including partial redistribution effects in angle and frequency of scattered photons (PRD). Furthermore, we include a regularization strategy that enables having model atmospheres with a complex depth stratification, without introducing artifacts. This approach takes steps towards a node-less inversion. In this paper we discuss the implementation of the aforementioned techniques, the description of the model atmosphere and the optimizations that we have applied to the code. We carry out some numerical experiments to show the performance of the code and of the regularization techniques that we have implemented. STiC has been made publicly available to the community.

arXiv:1810.08444v1 [pdf, other]
Elemental Abundance Analysis of Single and Binary Late-B Stars Using Sub-meter Class Telescopes: HR 342, HR 769, HR 1284, and HR 8705
Comments: 3 pages, 2 figures and 1 table. Accepted proceeding of the Observing techniques, instrumentation and science for metre-class telescopes II conference, September 24-28, 2018, Tatransk\'a Lomnica, Slovakia

We test the capabilities of 0.4 m telescopes equipped with an \'{e}chelle spectrograph to derive fundamental parameters and elemental abundances of four late-B type stars: HR 342, HR 769, HR 1284, and HR 8705. The medium resolution (R~14000) spectra covering the wavelength range of 4380-7350 {\AA} of the four stars have been obtained using the 40-cm-telescope in Ankara University Kreiken Observatory (AUKR). Using spectrum synthesis, we were able to derive the abundances of eleven chemical elements. We find that these stars do not show remarkable departures from the solar abundances, except for HR 8705 and the primary component of HR 1284, which exhibit slight underabundances of a few elements, i.e., O, Mg, Al, Si, and Fe. We also find that HR 1284 is probably a new spectroscopic binary star. In order to model the spectrum of this object, one of us (TK) has developed a new graphic interface which allows us to synthesize the composite spectrum of binary stars.

arXiv:1810.08449v1 [pdf, other]
The polytropic index of solar coronal plasma in sunspot fan loops and its temperature dependence
Comments: Accepted for publication in ApJ

Observations of slow magneto-acoustic waves have been demonstrated to possess a number of applications in coronal seismology. Determination of the polytropic index ($\gamma$) is one such important application. Analysing the amplitudes of oscillations in temperature and density corresponding to a slow magneto-acoustic wave, the polytropic index in the solar corona has been calculated and based on the obtained value it has been inferred that thermal conduction is highly suppressed in a very hot loop in contrast to an earlier report of high thermal conduction in a relatively colder loop. In this study, using SDO/AIA data, we analysed slow magneto-acoustic waves propagating along sunspot fan loops from 30 different active regions and computed polytropic indices for several loops at multiple spatial positions. The obtained $\gamma$ values vary from 1.04$\pm$0.01 to 1.58$\pm$0.12 and most importantly display a temperature dependence indicating higher $\gamma$ at hotter temperatures. This behaviour brings both the previous studies to agreement and perhaps implies a gradual suppression of thermal conduction with increase in temperature of the loop. The observed phase shifts between temperature and density oscillations, however, are substantially larger than that expected from a classical thermal conduction and appear to be influenced by a line-of-sight integration effect on the emission measure.

arXiv:1810.08450v1 [pdf, other]
Magnetic fields in massive spirals: The role of feedback and initial conditions

Magnetic fields play a very important role in the evolution of galaxies through their direct impact on star formation and stellar feedback-induced turbulence. However, their co-evolution with these processes has still not been thoroughly investigated, and the possible effect of the initial conditions is largely unknown. This letter presents the first results from a series of high-resolution numerical models, aimed at deciphering the effect of the initial conditions and of stellar feedback on the evolution of the galactic magnetic field in isolated, Milky-Way-like galaxies. The models start with an ordered, either poloidal or toroidal, magnetic field of varying strength, and are evolved with and without supernova feedback. They include a dark matter halo, a stellar and a gaseous disk, as well as the appropriate cooling and heating processes for the interstellar medium. Independently of the initial conditions, the galaxies develop a turbulent velocity field and a random magnetic field component in under 15 Myrs. Supernova feedback is extremely efficient in building a random magnetic field component up to large galactic heights. However, a random magnetic field emerges even in runs without feedback, which points to an inherent instability of the ordered component. Supernova feedback greatly affects the velocity field of the galaxy up to large galactic heights, and helps restructure the magnetic field up to 10 kpc above the disk, independently of the initial magnetic field morphology. On the other hand, the initial morphology of the magnetic field can accelerate the development of a random component at large heights. These effects have important implications for the study of the magnetic field evolution in galaxy simulations.

arXiv:1810.08453v1 [pdf, other]
A headless tadpole galaxy: the high gas-phase metallicity of the ultra-diffuse galaxy UGC 2162
Comments: Accepted for publication in ApJ. 10 pages. 5 figures

The cosmological numerical simulations tell us that accretion of external metal-poor gas drives star-formation (SF) in galaxy disks. One the best pieces of observational evidence supporting this prediction is the existence of low metallicity star-forming regions in relatively high metallicity host galaxies. The SF is thought to be fed by metal-poor gas recently accreted. Since the gas accretion is stochastic, there should be galaxies with all the properties of a host but without the low metallicity starburst. These galaxies have not been identified yet. The exception may be UGC 2162, a nearby ultra-diffuse galaxy (UDG) which combines low surface brightness and relatively high metallicity. We confirm the high metallicity of UGC 2162 (12 + log(O/H) = 8.52+0.27-0.24 ) using spectra taken with the 10-m GTC telescope. GC2162 has the stellar mass, metallicity, and star-formation rate (SFR) surface density expected for a host galaxy in between outbursts. This fact suggests a physical connection between some UDGs and metal-poor galaxies, which may be the same type of object in a different phase of the SF cycle. UGC 2162 is a high-metallicity outlier of the mass-metallicity relation, a property shared by the few UDGs with known gas-phase metallicity.

arXiv:1810.08454v1 [pdf, other]
Renormalization group computation of likelihood functions for cosmological data sets

I show how a renormalization group (RG) method can be used to incrementally integrate the information in cosmological large-scale structure data sets (including CMB, galaxy redshift surveys, etc.). I show numerical tests for Gaussian fields, where the method allows arbitrarily close to exact computation of the likelihood function in order $\sim N$ time, even for problems with no symmetry, compared to $N^3$ for brute force linear algebra (where $N$ is the number of data points -- to be fair, methods already exist to solve the Gaussian problem in at worst $N \log N$ time, and this method will not necessarily be faster in practice). The method requires no sampling or other Monte Carlo (random) element. Non-linearity/non-Gaussianity can be accounted for to the extent that terms generated by integrating out small scale modes can be projected onto a sufficient basis, e.g., at least in the sufficiently perturbative regime. The formulas to evaluate are straightforward and require no understanding of quantum field theory, but this paper may also serve as a pedagogical introduction to Wilsonian RG for astronomers.

arXiv:1810.08472v1 [pdf, other]
The Radial Acceleration Relation (RAR): the crucial cases of Dwarf Discs and of Low Surface Brightness galaxies

McGaugh et al. (2016) have found, in a large sample of disc systems, a tight nonlinear relationship between the total radial accelerations $g$ and their components $g_b$ arisen from the distribution of the baryonic matter ~\citep{McGaugh_2016}. Here, we investigate the existence of such relation in Dwarf Disk Spirals and Low Surface Brightness galaxies on the basis of ~\cite{Karukes_2017} and ~\cite{DiPaolo_2018}. We have accurate mass profiles for 36 Dwarf Disk Spirals and 72 LSB galaxies. These galaxies have accelerations that cover the McGaugh range but also reach one order of magnitude below the smallest accelerations present in McGaugh et al. (2016) and span different Hubble Types. We found, in our samples, that the $g$ vs $g_b$ relation has a very different profile and also other intrinsic novel properties among those the dependence on a second variable: the galactic radius normalised to the optical radius $R_{opt}$, at which the two accelerations are calculated. We show that the new complex $g$ vs $(g_b, r/R_{opt})$ relationship is not else that a direct consequence of the complex mass distribution in galaxies. Our analysis shows that the McGaugh et al. (2016) relation is a limiting case of our new complex universal relation and can be interpreted in the standard "DM halo in the Newtonian Gravity" paradigm.

arXiv:1810.08482v1 [pdf, other]
Fermi-LAT counterparts of IceCube neutrinos above 100 TeV
Comments: accepted for publication by A&A

The IceCube Collaboration has published four years of data and the observed neutrino flux is significantly in excess of the expected atmospheric background. Due to the steeply falling atmospheric background spectrum, events at the highest energies are most likely extraterrestrial. In our previous approach we have studied blazars as the possible origin of the High-Energy Starting Events (HESE) neutrino events at PeV energies. In this work we extend our study to include all HESE neutrinos (which does not include IC 170922A) at or above a reconstructed energy of 100 TeV, but below 1 PeV. We study the X-ray and $\gamma$-ray data of all ($\sim200$) 3LAC blazars that are positionally consistent with the neutrino events above 100 TeV to determine the maximum neutrino flux from these sources. This larger sample allows us to better constrain the scaling factor between the observed and maximum number of neutrino events. We find that when we consider a realistic neutrino spectrum and other factors, the number of neutrinos is in good agreement with the detected number of IceCube HESE events. We also show that there is no direct correlation between \Fermi-LAT $\gamma$-ray flux and the IceCube neutrino flux and that the expected number of neutrinos is consistent with the non-detection of individual bright blazars.

arXiv:1810.08499v1 [pdf, other]
Sensitivity of the KM3NeT/ARCA neutrino telescope to point-like neutrino sources
Comments: 32 pages, 10 figures, submitted to Astroparticle Physics

KM3NeT will be a network of deep-sea neutrino telescopes in the Mediterranean Sea. The KM3NeT/ARCA detector, to be installed at the Capo Passero site (Italy), is optimised for the detection of high-energy neutrinos of cosmic origin. Thanks to its geographical location on the Northern hemisphere, KM3NeT/ARCA can observe upgoing neutrinos from most of the Galactic Plane, including the Galactic Centre. Given its effective area and excellent pointing resolution, KM3NeT/ARCA will measure or significantly constrain the neutrino flux from potential astrophysical neutrino sources. At the same time, it will test flux predictions based on gamma-ray measurements and the assumption that the gamma-ray flux is of hadronic origin. Assuming this scenario, discovery potential and sensitivity to a selected list of Galactic sources and to generic point sources with an $E^{-2}$ spectrum are presented. These spectra are assumed to be time independent. The results indicate that an observation with $3\sigma$ significance is possible in about six years of operation for the most intense sources, such as Supernovae Remnants RX J1713.7-3946 and Vela Jr. If no signal will be found during this time, the fraction of the gamma-ray flux coming from hadronic processes can be constrained to be below 50% for these two objects.

arXiv:1810.08507v1 [pdf, other]
PKSB1740-517: An ALMA view of the cold gas feeding a distant interacting young radio galaxy
Comments: 15 pages, 7 figures, accepted for publication in MNRAS

Cold neutral gas is a key ingredient for growing the stellar and central black hole mass in galaxies throughout cosmic history. We have used the Atacama Large Millimetre Array (ALMA) to detect a rare example of redshifted $^{12}$CO(2-1) absorption in PKS B1740-517, a young ($t \sim 1.6 \times 10^{3}$ yr) and luminous ($L_{\rm 5 GHz} \sim 6.6 \times 10^{43}$ erg s$^{-1}$ ) radio galaxy at $z = 0.44$ that is undergoing a tidal interaction with at least one lower-mass companion. The coincident HI 21-cm and molecular absorption have very similar line profiles and reveal a reservoir of cold gas ($M_{\rm gas} \sim 10^{7} - 10^{8}$ M$_{\odot}$), likely distributed in a disc or ring within a few kiloparsecs of the nucleus. A separate HI component is kinematically distinct and has a very narrow line width ($\Delta{v}_{\rm FWHM} \lesssim 5$ km s$^{-1}$), consistent with a single diffuse cloud of cold ($T_{\rm k} \sim 100$ K) atomic gas. The $^{12}$CO(2-1) absorption is not associated with this component, which suggests that the cloud is either much smaller than 100 pc along our sight-line and/or located in low-metallicity gas that was possibly tidally stripped from the companion. We argue that the gas reservoir in PKS B1740-517 may have accreted onto the host galaxy $\sim$50 Myr before the young radio AGN was triggered, but has only recently reached the nucleus. This is consistent with the paradigm that powerful luminous radio galaxies are triggered by minor mergers and interactions with low-mass satellites and represent a brief, possibly recurrent, active phase in the life cycle of massive early type galaxies.

arXiv:1810.08513v1 [pdf, other]
Indirect Detection of Extrasolar Planets via Astrometry

Radio wavelength astrometry of stars and other objects has a long and productive history. The use of that technique to determine whether stars have planets around them would cover a nearly unique part of the parameter space for detection of those systems. Namely, astrometric observations are most sensitive to systems with large planets in moderately wide orbits (a few to ~10 AU), because it is those systems that produce large reflex motion of the star, in a short enough measurement period (years to tens of years). In addition, astrometric observations are most sensitive to systems which are nearly face-on. Other techniques (radial velocity, or the photometric method of Kepler) are more sensitive to systems with planets in close orbits (less than $\sim$1 AU), which are nearly edge-on. We describe here, using the Hipparcos and Gaia star catalogs, how ngVLA could use this technique on hundreds of stars, some tens of which are solar analogs, to determine whether these stars have planets orbiting them.

arXiv:1810.08519v1 [pdf, other]
Using bootstrap to assess uncertainties of VLBI results I. The method and image-based errors
Comments: Accepted for publication in MNRAS. 11 pages, 16 figures

Very Long Baseline Interferometric (VLBI) observations of quasar jets enable one to measure many theoretically expected effects. Estimating the significance of observational findings is complicated by the correlated noise in the image plane. A reliable and well justified approach to estimate the uncertainties of VLBI results is needed as well as significance testing criteria. We propose to use bootstrap for both tasks. Using simulations we find that bootstrap-based errors for the full intensity, rotation measure, and spectral index maps have coverage closer to the nominal values than conventionally obtained errors. The proposed method naturally takes into account heterogeneous interferometric arrays (such as Space VLBI) and can be easily extended to account for instrumental calibration factors.

arXiv:1810.08521v1 [pdf, other]
Potential for Solar System Science with the ngVLA

Radio wavelength observations of solar system bodies are a powerful method of probing many characteristics of those bodies. From surface and subsurface, to atmospheres (including deep atmospheres of the giant planets), to rings, to the magnetosphere of Jupiter, these observations provide unique information on current state, and sometimes history, of the bodies. The ngVLA will enable the highest sensitivity and resolution observations of this kind, with the potential to revolutionize our understanding of some of these bodies. In this article, we present a review of state-of-the-art radio wavelength observations of a variety of bodies in our solar system, varying in size from ring particles and small near-Earth asteroids to the giant planets. Throughout the review we mention improvements for each body (or class of bodies) to be expected with the ngVLA. A simulation of a Neptune-sized object is presented in Section 6. Section 7 provides a brief summary for each type of object, together with the type of measurements needed for all objects throughout the Solar System.

arXiv:1810.08526v1 [pdf, other]
Analysis of HST, VLT and Gemini coordinated observations of Uranus late 2017 : a multi-spectral search for auroral signatures

On 6 Sept. 2017, an exceptional coronal mass ejection departed from the Sun toward the Earth and Uranus, whose magnetospheres are sensitive to the solar wind. The resulting interplanetary shock triggered geomagnetic storm and intense aurora at Earth the next day and was predicted by MHD models to reach Uranus around 10-11 Nov. This event provided a unique opportunity to investigate the auroral response of the asymmetric Uranian magnetosphere in its intermediate equinox-to-solstice configuration. Coordinated multi-spectral observations were acquired with the Hubble Space Telescope (HST) in the far-UV (FUV), with the Very Large Telescope (VLT) and Gemini North in the near-IR (NIR) and with Chandra in the X-ray domain. In this study, we focus on the analysis of NIR images obtained between 9 and 17 Nov. 2017 which are compared to one FUV image acquired on 11 Nov. The latter reveals a bright southern auroral spot in the H2 bands, which we use as a reference to locate auroral precipitations. The NIR images were aimed at mapping H3+ emission from the Uranian ionosphere and at updating the results built from a couple of pioneer images taken 25 years ago. These new high resolution images reveal H3+ from the whole disc although brighter near the southern pole, but show no evidence of localized auroral emission.

arXiv:1810.08543v1 [pdf, other]
Strong Constraints on Fuzzy Dark Matter from Ultrafaint Dwarf Galaxy Eridanus II

The fuzzy dark matter (FDM) model treats DM as a bosonic field with astrophysically large de Broglie wavelength. A striking feature of this model is $\mathcal{O}(1)$ fluctuations in the dark matter density on time scales which are shorter than the gravitational timescale. Including for the first time the effect of core oscillations, we demonstrate how such fluctuations lead to heating of star clusters, and thus an increase in their size over time. From the survival of the old star cluster in Eridanus II we infer $m_a\gtrsim 0.6\rightarrow 1\times 10^{-19}\text{ eV}$ within modelling uncertainty if FDM is to compose all of the DM, and derive constraints on the FDM fraction at lower masses. The subhalo mass function in the Milky Way implies $m_a\gtrsim 0.8\times 10^{-21}\text{ eV}$ to successfully form Eridanus II. The window between $10^{-21}\text{ eV}\lesssim m_a\lesssim 10^{-20}\text{ eV}$ is affected by narrow band resonances, and the limited applicability of the diffusion approximation. Some of this window may be consistent with observations of Eridanus II and more detailed investigations are required.

arXiv:1810.08548v1 [pdf, other]
The Rotational Evolution of Young, Binary M Dwarfs

We have analysed K2 light curves for more than 3,000 low mass stars in the $\sim$8 Myr old Upper Sco association, the $\sim$125 Myr age Pleiades open cluster and the $\sim$700 Myr old Hyades and Praesepe open clusters to determine stellar rotation rates. Many of these K2 targets show two distinct periods, and for the lowest mass stars in these clusters virtually all of these systems with two periods are photometric binaries. The most likely explanation is that we are detecting the rotation periods for both components of these binaries. We explore the evolution of the rotation rate in both components of photometric binaries relative to one another and to non-photometric binary stars. In Upper Sco and the Pleiades, these low mass binary stars have periods that are much shorter on average and much closer to each other than would be true if drawn at random from the M dwarf single stars. In Upper Sco, this difference correlates strongly with the presence or absence of infrared excesses due to primordial circumstellar disks -- the single star population includes many stars with disks, and their rotation periods are distinctively longer on average than their binary star cousins of the same mass. By Praesepe age, the significance of the difference in rotation rate between the single and binary low mass dMs is much less, suggesting that angular momentum loss from winds for fully-convective zero-age main sequence stars erases memory of the rotation rate dichotomy for binary and single very low mass stars at later ages.

arXiv:1810.08576v1 [pdf, other]
Probing the IGMF with the next generation Cherenkov telescopes
Comments: Accepted for publication by ApJ, 13 pages, 13 figures

Intergalactic space is believed to contain non-zero magnetic fields (the Intergalactic Magnetic Field: IGMF) which at scales of Mpc would have intensities below $10^{-9}$ G. Very high energy (VHE $>$100 GeV) gamma rays coming from blazars can produce e$^+$e$^-$ pairs when interacting with the Extragalactic Background Light (EBL) and the Cosmic Microwave Background, generating an electromagnetic cascade of Mpc scale. The IGMF may produce a detectable broadening of the emission beam that could lead to important constrains both on the IGMF intensity and its coherence length. Using the Monte Carlo-based Elmag code, we simulate the electromagnetic cascade corresponding to two detected TeV sources: PKS 2155-304 visible from the South and H1426+428 visible from the North. Assuming an EBL model and intrinsic spectral properties of the sources we obtain the spectral and angular distribution of photons when they arrive at Earth. We include the response of the next generation Cherenkov telescopes by using simplified models for CTA (Cherenkov Telescope Array)-south and CTA-north based on a full simulation of each array performance. Combining the instrument properties with the simulated source fluxes, we calculate the telescope point spread function for null and non-null IGMF intensities and develop a method to test the statistical feasibility of detecting IGMF imprints by comparing the resulting angular distributions. Our results show that for the analysed source PKS 2155-304 corresponding to the southern site, CTA should be able to detect IGMF with intensities stronger than 10$^{-14.5}$G within an observation time of $\sim$100 hours.

arXiv:1810.08583v1 [pdf, other]
Principal component analysis of sunspot cycle shape
Comments: 10 pages, 16 figures, accepted to A&A

We study the shape of sunspot cycles using the Wolf sunspot numbers and group sunspot numbers of solar cycles 1-23. We determine the most typical "model" cycles and the most asymmetric cycles, and test the validity of the two Waldmeier rules: the anti-correlation between cycle height and the length of its ascending phase (rule 1), and between cycle height and the length of the preceding cycle (rule 2). We applied the principal component analysis to sunspot cycles and studied the first two components, which describe the average cycle shape and cycle asymmetry, respectively. We also calculated their autocorrelation in order to study their recurrence properties. The best model cycles for Wolf numbers are SC12, SC14, and SC16, the successive even cycles from a long period of rather low overall solar activity. We find that the model cycles in eight different analyses using both sunspot series are almost exclusively even cycles. Correspondingly, the most asymmetric cycles are odd cycles. We find that both Waldmeier rules are valid for the whole Wolf number series of 23 cycles. Waldmeier rule 2 is also valid for group number series although its significance is weaker. Waldmeier rule 1 is not significant for the original group number series, but becomes significant for the proxy series. For separate centuries, Waldmeier rules are not always valid for Wolf numbers and very rarely for group numbers. Our results also offer a new interpretation for the Gnevyshev gap. In addition to being a local depression of solar activity, the Gnevyshev gap is a separatrix that divides cycles into two parts whose relative intensities determine the cycle asymmetry. The Gnevyshev gap is the zero value time of PC2, located approximately 33-42% into the cycle after its start.