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[1]  arXiv:1602.03524 [pdf, ps, other]
Title: Constraining the subgrid physics in simulations of isolated dwarf galaxies
Authors: Bert Vandenbroucke, Robbert Verbeke, Sven De Rijcke (Ghent University)
Comments: Accepted for publication in MNRAS. 25 pages, 2 tables and 36 figures. Interactive plots can be found on this http URL
Subjects: Astrophysics of Galaxies (astro-ph.GA)

Simulating dwarf galaxy halos in a reionizing Universe puts severe constraints on the sub-grid model employed in the simulations. Using the same sub-grid model that works for simulations without a UV-background (UVB) results in gas poor galaxies that stop forming stars very early on, except for halos with high masses. This is in strong disagreement with observed galaxies, which are gas rich and star forming down to a much lower mass range. To resolve this discrepancy, we ran a large suite of isolated dwarf galaxy simulations to explore a wide variety of sub-grid models and parameters, including timing and strength of the UVB, strength of the stellar feedback, and metallicity dependent Pop III feedback. We compared these simulations to observed dwarf galaxies by means of the baryonic Tully-Fisher relation (BTFR), which links the baryonic content of a galaxy to the observationally determined strength of its gravitational potential. We found that the results are robust to changes in the UVB. The strength of the stellar feedback shifts the results on the BTFR, but does not help to form gas rich galaxies at late redshifts. Only by including Pop III feedback are we able to produce galaxies that lie on the observational BTFR and that have neutral gas and ongoing star formation at redshift zero.

[2]  arXiv:1602.03526 [pdf, ps, other]
Title: Cold Dark Matter Substructures in Early-Type Galaxy Halos
Authors: Davide Fiacconi (1), Piero Madau (1,2,3), Doug Potter (1), Joachim Stadel (1) ((1) Institute for Computational Science, University of Zurich, (2) Institute for Astronomy, ETH Zurich, (3) Departmente of Astronomy and Astrophysics, UCSC)
Comments: 14 pages, 14 figures, submitted to ApJ, comments are welcome
Subjects: Astrophysics of Galaxies (astro-ph.GA); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

We present initial results from the "Ponos" zoom-in numerical simulations of dark matter substructures in massive ellipticals. Two very highly resolved dark matter halos with $M_{\rm vir}=1.2\times 10^{13}$ $M_{\odot}$ and $M_{\rm vir}=6.5\times 10^{12}$ $M_{\odot}$ and different ("violent" vs. "quiescent") assembly histories have been simulated down to $z=0$ in a $\Lambda$CDM cosmology with a total of 921,651,914 and 408,377,544 particles, respectively. Within the virial radius, the total mass fraction in self-bound $M_{\rm sub}>10^6$ $M_{\odot}$ subhalos at the present epoch is 15% for the violent host and 16.5% for the quiescent one. At $z=0.7$, these fractions increase to 19 and 33%, respectively, as more recently accreted satellites are less prone to tidal destruction. In projection, the average fraction of surface mass density in substructure at a distance of $R/R_{\rm vir}=0.02$ ($\sim 5-10$ kpc) from the two halo centers ranges from 0.6% to $\gtrsim 2$%, significantly higher than measured in simulations of Milky Way-sized halos. The contribution of subhalos with $M_{\rm sub} < 10^9$ $M_{\odot}$ to the projected mass fraction is between one fifth and one third of the total, with the smallest share found in the quiescent host. We assess the impact of baryonic effects via twin, lower-resolution hydrodynamical simulations that include metallicity-dependent gas cooling, star formation, and a delayed-radiative-cooling scheme for supernova feedback. Baryonic contraction produces a super-isothermal total density profile and increases the number of massive subhalos in the inner regions of the main host. The host density profiles and projected subhalo mass fractions appear to be broadly consistent with observations of gravitational lenses.

[3]  arXiv:1602.03527 [pdf, other]
Title: Can Star-Disk Collisions Explain the Missing Red Giants Problem in the Galactic Center?
Authors: Thomas Forrest Kieffer, Tamara Bogdanovic (Georgia Institute of Technology)
Comments: 15 pages, 11 figures, submitted to ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

Observations have revealed a relative paucity of red giant (RG) stars within the central 0.5pc in the Galactic Center (GC). Motivated by this finding we investigate the hypothesis that collisions of stars with a fragmenting accretion disk are responsible for the observed dearth of evolved stars. We use 3-dimensional hydrodynamic simulations to model a star with radius $10 R_{\odot}$ and mass $1 M_{\odot}$, representative of the missing population of RGs, colliding with high density clumps. We find that multiple collisions with clumps of column density $\gtrsim10^{8}\, {\rm g\,cm^{-2}}$ can strip a substantial fraction of the star's envelope and in principle render it invisible to observations. Simulations confirm that repeated impacts are particularly efficient in driving mass loss as partially stripped RGs expand and have increased cross sections for subsequent collisions. Because the envelope is unbound on account of the kinetic energy of the star, any significant amount of stripping of the RG population in the GC should be mirrored by a systematic decay of their orbits and possibly by their enhanced rotational velocity. To be viable, this scenario requires that the total mass of the fragmenting disk has been several orders of magnitude higher than that of the early type stars which now form the stellar disk in the GC.

[4]  arXiv:1602.03528 [pdf, ps, other]
Title: The featureless and non-variable optical spectral energy distribution of AXP 4U 0142+61
Comments: Accepted for publication in MNRAS Letters
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)

We present GTC-10.4m spectroscopy and multi-band photometry of the faint (r ~26) optical counterpart of the anomalous X-ray pulsar 4U 0142+61. The 5000 - 9000 Angs spectrum -- the first obtained for a magnetar -- is featureless, allowing us to set an equivalent width upper limit EW < 25 Angs to the presence of emission lines in the Halfa region. Multi-band photometry in the g, r, i, z SDSS bands obtained at different epochs over 12 years shows no significant variability from minutes-to-years time scales. The photometry has been calibrated, for the first time, against the SDSS itself, resulting in solid upper limits to variability ranging from ~0.2 mag in i (over 12 years) to 0.05 mag in z (over 1.5 years). The shape of the optical + near-infrared (literature values) spectral energy distribution is not well constrained due to the high extinction along the line of sight. Using a Markov Chain Monte Carlo analysis we find that it can be described by a power-law with a spectral index beta=-0.7+-0.5 and E_(B-V)=1.5+-0.4. We also discuss on the implications of adding hard X-ray flux values from literature to the spectral fitting.

[5]  arXiv:1602.03531 [pdf, other]
Title: G11.2-0.3: The Young Remnant of a Stripped-Envelope Supernova
Comments: 20 pages, 14 figures, accepted for publication in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

We present results of a 400-ks Chandra observation of the young shell supernova remnant (SNR) G11.2-0.3, containing a pulsar and pulsar-wind nebula (PWN). We measure a mean expansion rate for the shell since 2000 of 0.0277+/-0.0018% per yr, implying an age between 1400 and 2400 yr, and making G11.2-0.3 one of the youngest core-collapse SNRs in the Galaxy. However, we find very high absorption ($A_V \sim 16^m \pm 2^m$), confirming near-IR determinations and ruling out a claimed association with the possible historical SN of 386 CE. The PWN shows strong jets and a faint torus within a larger, more diffuse region of radio emission and nonthermal X-rays. Central soft thermal X-ray emission is anticorrelated with the PWN; that, and more detailed morphological evidence, indicates that the reverse shock has already reheated all ejecta and compressed the PWN. The pulsar characteristic energy-loss timescale is well in excess of the remnant age, and we suggest that the bright jets have been produced since the recompression. The relatively pronounced shell and diffuse hard X-ray emission in the interior, enhanced at the inner edge of the shell, indicate that the immediate circumstellar medium into which G11.2-0.3 is expanding was quite anisotropic. We propose a possible origin for G11.2-0.3 in a stripped-envelope progenitor that had lost almost all its envelope mass, in an anisotropic wind or due to binary interaction, leaving a compact core whose fast winds swept previously lost mass into a dense irregular shell, and which exploded as a Type cIIb or Ibc supernova.

[6]  arXiv:1602.03532 [pdf, ps, other]
Title: Peering through the Dust: NuSTAR Observations of Two FIRST-2MASS Red Quasars
Comments: accepted for publication to ApJ; 34 pages, 8 figures, 5 tables
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics of Galaxies (astro-ph.GA)

Some reddened quasars appear to be transitional objects in the merger-induced black hole growth/galaxy evolution paradigm, where a heavily obscured nucleus starts to be unveiled by powerful quasar winds evacuating the surrounding cocoon of dust and gas. Hard X-ray observations are able to peer through this gas and dust, revealing the properties of circumnuclear obscuration. Here, we present NuSTAR and XMM-Newton/Chandra observations of FIRST-2MASS selected red quasars F2M 0830+3759 and F2M 1227+3214. We find that though F2M 0830+3759 is moderately obscured ($N_{\rm H,Z} = 2.1\pm0.2 \times10^{22}$ cm$^{-2}$) and F2M 1227+3214 is mildly absorbed ($N_{\rm H,Z} = 3.4^{+0.8}_{-0.7}\times10^{21}$ cm$^{-2}$) along the line-of-sight, heavier global obscuration may be present in both sources, with $N_{\rm H,S} = 3.7^{+4.1}_{-2.6} \times 10^{23}$ cm$^{-2}$ and $< 5.5\times10^{23}$ cm$^{-2}$, for F2M 0830+3759 and F2M 1227+3214, respectively. F2M 0830+3759 also has an excess of soft X-ray emission below 1 keV which is well accommodated by a model where 7% of the intrinsic AGN X-ray emission is scattered into the line-of-sight. While F2M 1227+3214 has a dust-to-gas ratio ($E(B-V)$/$N_{\rm H}$) consistent with the Galactic value, the $E(B-V)$/$N_{\rm H}$ value for F2M 0830+3759 is lower than the Galactic standard, consistent with the paradigm that the dust resides on galactic scales while the X-ray reprocessing gas originates within the dust-sublimation zone of the broad-line-region. The X-ray and 6.1$\mu$m luminosities of these red quasars are consistent with the empirical relations derived for high-luminosity, unobscured quasars, extending the parameter space of obscured AGN previously observed by NuSTAR to higher luminosities.

[7]  arXiv:1602.03533 [pdf, ps, other]
Title: Diffuse Low-Ionization Gas in the Galactic Halo Casts Doubts on $z\simeq 0.03$ WHIM Detections
Authors: F. Nicastro (1,2,3), F. Senatore (1), A. Gupta (4,5), S. Mathur (4,6), Y. Krongold (7), M. Elvis (2), L. Piro (8) ((1) Osservatorio Astronomico di Roma - INAF, Roma, Italy, (2) Harvard-Smithsonian Center for Astrophysics, Cambridge, MA, USA, (3) University of Crete, Heraklion, Greece, (4) Ohio State University, Columbus, OH, USA, (5) Columbus State Community college, Columbus, OH, USA, (6) Center for Cosmology and Astro-Particle Physics (CCAPP), The Ohio State University, OH, USA, (7) Instituto de Astronomia, Universidad Nacional Autonoma de Mexico, Mexico City, Mexico, (8) Istituto di Astrofisica e Planetologia Spaziali - INAF, Roma, Italy)
Comments: 6 pages, 2 figures, accepted by MNRAS (Letter)
Subjects: Astrophysics of Galaxies (astro-ph.GA)

In this Letter we demonstrate that the two claims of $z\simeq 0.03$ OVII K$\alpha$ absorption lines from Warm Hot Intergalactic Medium (WHIM) along the lines of sight to the blazars H~2356-309 (Buote et al., 2009; Fang et al., 2010) and Mkn~501 (Ren, Fang \& Buote, 2014) are likely misidentifications of the $z=0$ OII K$\beta$ line produced by a diffuse Low-Ionization Metal Medium in the Galaxy's Interstellar and Circum-Galactic mediums. We perform detailed modeling of all the available high signal-to-noise Chandra LETG and XMM-Newton RGS spectra of H 2356-309 and Mkn 501 and demonstrate that the $z\simeq 0.03$ WHIM absorption along these two sightlines is statistically not required. Our results, however, do not rule out a small contribution from the $z\simeq 0.03$ OVII K$\alpha$ absorber along the line of sight to H~2356-309. In our model the temperature of the putative $z = 0.031$ WHIM filament is T$= 3\times 10^5$ K and the OVII column density is N$_{OV II} \le 4\times 10^{15}$ cm$^{-2}$, twenty times smaller than the OVII column density previously reported, and now more consistent with the expectations from cosmological hydrodynamical simulations.

[8]  arXiv:1602.03545 [pdf, other]
Title: On the rate and on the gravitational wave emission of short and long GRBs
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Short and long-duration gamma-ray bursts (GRBs) are expected to originate in binary systems. For short bursts, we have as possible progenitors neutron star (NS) binaries (NS-NS), or of NS-black hole (BH) binaries (NS-BH). For long bursts, the induced gravitational collapse (IGC) paradigm introduces a tight binary system composed of a carbon-oxygen core (CO$_{\rm core}$) undergoing supernova (SN) explosion that in turn triggers a hypercritical accretion process onto a NS companion, or onto an already formed BH. These IGC events lead lead either to NS-NS or to NS-BH binaries, the latter formed when the accretion is sufficient to bring the NS to its critical mass. We use recent estimates of the observed rates in X and gamma-rays of the above systems to estimate the gravitational wave emission in all these binaries and their expected detection rate by Advanced LIGO.

[9]  arXiv:1602.03558 [pdf, other]
Title: Non-local gravity and comparison with observational datasets. II. Updated results and Bayesian model comparison with $Λ$CDM
Comments: 41 pages, 18 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)

We present a comprehensive and updated comparison with cosmological observations of two non-local modifications of gravity previously introduced by our group, the so called RR and RT models. We implement the background evolution and the cosmological perturbations of the models in a modified Boltzmann code, using CLASS. We then test the non-local models against the Planck 2015 TT, TE, EE and Cosmic Microwave Background (CMB) lensing data, isotropic and anisotropic Baryonic Acoustic Oscillations (BAO) data, JLA supernovae, $H_0$ measurements and growth rate data, and we perform Bayesian parameter estimation. We then compare the RR, RT and $\Lambda$CDM models, using the Savage-Dickey method. We find that the RT model and $\Lambda$CDM perform equally well, while the RR model is disfavored.

[10]  arXiv:1602.03562 [pdf, ps, other]
Title: The large-scale correlations of multi-cell densities and profiles, implications for cosmic variance estimates
Comments: 16 pages, 9 figures, submitted to MNRAS
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

In order to quantify the error budget in the measured probability distribution functions of cell densities, the two-point statistics of cosmic densities in concentric spheres is investigated. Bias functions are introduced as the ratio of their two-point correlation function to the two-point correlation of the underlying dark matter distribution. They describe how cell densities are spatially correlated. They are computed here via the so-called large deviation principle in the quasi-linear regime. Their large-separation limit is presented and successfully compared to simulations for density and density slopes: this regime is shown to be rapidly reached allowing to get sub-percent precision for a wide range of densities and variances. The corresponding asymptotic limit provides an estimate of the cosmic variance of standard concentric cell statistics applied to finite surveys. More generally, no assumption on the separation is required for some specific moments of the two-point statistics, for instance when predicting the generating function of cumulants containing any powers of concentric densities in one location and one power of density at some arbitrary distance from the rest. This exact "one external leg" cumulant generating function is used in particular to probe the rate of convergence of the large-separation approximation.

[11]  arXiv:1602.03628 [pdf, ps, other]
Title: Nonlinear reflection process of linearly-polarized, broadband Alfvén waves in the fast solar wind
Comments: 31 pages, 12 figures, accepted by The Astrophysical Journal
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Using one-dimensional numerical simulations, we study the elementary process of Alfv\'{e}n wave reflection in a uniform medium, including nonlinear effects. In the linear regime, Alfv\'{e}n wave reflection is triggered only by the inhomogeneity of the medium, whereas in the nonlinear regime, it can occur via nonlinear wave-wave interactions. Such nonlinear reflection (backscattering) is typified by decay instability. In most studies of decay instabilities, the initial condition has been a circularly polarized Alfv\'{e}n wave. In this study we consider a linearly polarized Alfv\'en wave, which drives density fluctuations by its magnetic pressure force. For generality, we also assume a broadband wave with a red-noise spectrum. In the data analysis, we decompose the fluctuations into characteristic variables using local eigenvectors, thus revealing the behaviors of the individual modes. Different from circular-polarization case, we find that the wave steepening produces a new energy channel from the parent Alfv\'en wave to the backscattered one. Such nonlinear reflection explains the observed increasing energy ratio of the sunward to the anti-sunward Alfv\'{e}nic fluctuations in the solar wind with distance against the dynamical alignment effect.

[12]  arXiv:1602.03629 [pdf, ps, other]
Title: TeV gamma-ray emission initiated by the population or individual millisecond pulsars within globular clusters
Comments: 15 pages, submitted to MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Two energetic millisecond pulsars (MSPs) within globular clusters (GC), J1823-3021A in NGC 6624 and PSR B1821-24 in M28, have been recently discovered to emit pulsed GeV gamma-rays. These MSPs are expected to eject energetic leptons. Therefore, GCs have been proposed to produce GeV-TeV gamma-rays as a result of the comptonization process of the background radiation within a GC. We develop this general scenario by taking into account not only the diffusion process of leptons within a GC but also their advection with the wind from the GC. Moreover, we consider distribution of MSP within a GC and the effects related to the non-central location of the dominating, energetic MSP. Such more complete scenario is considered for the modelling of the GeV-TeV gamma-ray emission from the core collapsed GC M15 and also for GCs which contain recently discovered energetic MSPs within NGC 6624 and M28. The confrontation of the modelling of the gamma-ray emission with the observations with the present Cherenkov telescopes and the future Cherenkov Telescope Array (CTA) allows to constrain more reliably the efficiency of lepton production within the inner magnetosphere of the MSPs and re-accelerated in their vicinity. We discuss the expected limits on this parameter in the context of expectations from the pulsar models. we conclude that deep observations of GCs, even with the present sensitivity of Cherenkov telescopes (H.E.S.S., MAGIC, VERITAS), should start to constrain the models for the acceleration and radiation processes of leptons within the inner pulsar magnetosphere and its surrounding.

[13]  arXiv:1602.03634 [pdf, ps, other]
Title: Directed searches for broadband extended gravitational-wave emission in nearby energetic core-collapse supernovae
Comments: 26 pages, 8 figures, to appear in ApJ
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Core-collapse supernovae are factories of neutron stars and stellar mass black holes. Type Ib/c supernovae stand out as potentially originating in relatively compact stellar binaries and their branching ratio of about 1\% into long gamma-ray bursts. The most energetic events probably derive from central engines harboring rapidly rotating black holes, wherein accretion of fall-back matter down to the Inner Most Stable Circular Orbit (ISCO) offers a window to {\em broadband extended gravitational-wave emission} (BEGE). To search for BEGE, we introduce a butterfly filter in time-frequency space by Time Sliced Matched Filtering. To analyze long epochs of data, we propose using coarse grained searches followed by high resolution searches on events of interest. We illustrate our proposed coarse grained search on two weeks of LIGO S6 data prior to SN 2010br $(z=0.002339)$ using a bank of up to 64 thousand templates of one second duration covering a broad range in chirp frequencies and bandwidth. Correlating events with signal-to-noise ratios $>6$ from the LIGO L1 and H1 detectors each reduces to a few events of interest. Lacking any further properties reflecting a common excitation by broadband gravitational radiation, we disregarded these as spurious. This new pipeline may be used to systematically search for long duration chirps in nearby core-collapse supernovae from robotic optical transient surveys using embarrassingly parallel computing.

[14]  arXiv:1602.03640 [pdf, ps, other]
Title: The International Pulsar Timing Array: First Data Release
Comments: 25 pages, 6 tables, 5 figures. Accepted for publication in MNRAS
Subjects: Instrumentation and Methods for Astrophysics (astro-ph.IM)

The highly stable spin of neutron stars can be exploited for a variety of (astro-)physical investigations. In particular arrays of pulsars with rotational periods of the order of milliseconds can be used to detect correlated signals such as those caused by gravitational waves. Three such "Pulsar Timing Arrays" (PTAs) have been set up around the world over the past decades and collectively form the "International" PTA (IPTA). In this paper, we describe the first joint analysis of the data from the three regional PTAs, i.e. of the first IPTA data set. We describe the available PTA data, the approach presently followed for its combination and suggest improvements for future PTA research. Particular attention is paid to subtle details (such as underestimation of measurement uncertainty and long-period noise) that have often been ignored but which become important in this unprecedentedly large and inhomogeneous data set. We identify and describe in detail several factors that complicate IPTA research and provide recommendations for future pulsar timing efforts. The first IPTA data release presented here (and available online) is used to demonstrate the IPTA's potential of improving upon gravitational-wave limits placed by individual PTAs by a factor of ~2 and provides a 2-sigma limit on the dimensionless amplitude of a stochastic GWB of 1.7x10^{-15} at a frequency of 1 yr^{-1}. This is 1.7 times less constraining than the limit placed by (Shannon et al. 2015), due mostly to the more recent, high-quality data they used.

[15]  arXiv:1602.03665 [pdf]
Title: History and Development of Coronal Mass Ejections as a Key Player in Solar Terrestrial Relationship
Authors: Nat Gopalswamy
Comments: 33 pages, 11 figures, accepted for publication in Geoscience Letters February 10, 2016
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Coronal mass ejections (CMEs) are relatively a recently-discovered phenomenon, in 1971, some fifteen years into the Space Era. It took another two decades to realize that CMEs are the most important players in solar terrestrial relationship as the root cause of severe weather in Earth's space environment. CMEs are now counted among the major natural hazards because they cause large solar energetic particle (SEP) events and major geomagnetic storms, both of which pose danger to humans and their technology in space and ground. Geomagnetic storms discovered in the 1700s, solar flares discovered in the 1800s, and SEP events discovered in the1900s are all now found to be closely related to CMEs via various physical processes occurring at various locations in and around CMEs, when they interact with the ambient medium. This article identifies a number of key developments that preceded the discovery of white-light CMEs suggesting that CMEs were waiting to be discovered. The last two decades witnessed an explosion of CME research following the launch of the Solar and Heliospheric Observatory mission in 1995, resulting in the establishment of a full picture of CMEs.

[16]  arXiv:1602.03666 [pdf]
Title: The Sun and its Planets as detector for invisible matter
Comments: 9 pages 7 Figures
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

Gravitational lensing / deflection of invisible streaming matter towards the Sun could explain the puzzling solar flares and the unexplained solar emission in the EUV. Assuming that this invisible matter has some form of interaction with normal matter and that there exist preferred directions in its flow, then one would expect a more pronounced solar activity at certain planetary heliocentric longitudes. This is best demonstrated in the case of the 3 inner planets, considering their relatively short revolution time in comparison to a solar cycle of about 11 years. We have analyzed the solar flares as well as the EUV emission. We observe statistically significant signals when one or more planets have heliocentric longitudes mainly between 230o and 300o. The broad velocity spectrum of the assumed constituents makes it at this stage difficult to identify its source(s) in space. More refined future analyses might increase the precision of the determination of the stream direction and some properties of its constituents.

[17]  arXiv:1602.03667 [pdf, other]
Title: Asteroseismology of red-giant stars as a novel approach in the search for gravitational waves
Comments: Submitted to Astronomy in Focus, to appear in the Proceedings of the IAU XXIX General Assembly; 2 pages, 1 figure
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); General Relativity and Quantum Cosmology (gr-qc)

Stars are massive resonators that may be used as gravitational-wave (GW) detectors with isotropic sensitivity. New insights on stellar physics are being made possible by asteroseismology, the study of stars by the observation of their natural oscillations. The continuous monitoring of oscillation modes in stars of different masses and sizes (e.g., as carried out by NASA's Kepler mission) opens the possibility of surveying the local Universe for GW radiation. Red-giant stars are of particular interest in this regard. Since the mean separation between red giants in open clusters is small (a few light years), this can in principle be used to look for the same GW imprint on the oscillation modes of different stars as a GW propagates across the cluster. Furthermore, the frequency range probed by oscillations in red giants complements the capabilities of the planned eLISA space interferometer. We propose asteroseismology of red giants as a novel approach in the search for gravitational waves.

[18]  arXiv:1602.03668 [pdf, ps, other]
Title: Virilization of the Broad Line Region in Active Galactic Nuclei - connection between shifts and widths of broad emission lines
Comments: 24 pages, 8 figures, 4 tables, accepted in Astrophysics and Space Science
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We investigate the virilization of the emission lines Hbeta and Mg II in the sample of 287 Type 1 Active Galactic Nuclei taken from the Sloan Digital Sky Survey database. We explore the connections between the intrinsic line shifts and full widths at different levels of maximal intensity. We found that: (i) Hbeta seems to be a good virial estimator of black hole masses, and an intrinsic redshift of Hbeta is dominantly caused by the gravitational effect, (ii) there is an anti-correlation between the redshift and width of the wings of the Mg II line, (iii) the broad Mg II line can be used as virial estimator only at 50% of the maximal intensity, while the widths and intrinsic shifts of the line wings can not be used for this purpose.

[19]  arXiv:1602.03680 [pdf, other]
Title: The spectroscopic imprint of the pre-eruptive configuration resulting into two major coronal mass ejections
Comments: 14 pages, 12 figures, 4 tables
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

We present a spectroscopic analysis of the pre-eruptive configuration of active region NOAA 11429, prior to two very fast coronal mass ejections (CMEs) on March 7, 2012 that are associated with this active region. We study the thermal components and the dynamics associated with the ejected flux ropes. Using differential emission measure (DEM) analysis of Hinode/EIS and SDO/AIA observations, we identify the emission components of both the flux rope and the host active region. We then follow the time evolution of the flux rope emission components by using AIA observations. The plasma density and the Doppler and non-thermal velocities associated with the flux ropes are also calculated from the EIS data. The eastern and western parts of the active region, in which the two different fast CMEs originated during two X-class flares, were studied separately. In both regions we identified an emission component in the temperature range of $\log T=6.8 - 7.1$ associated with the presence of flux ropes. The time evolution of the eastern region showed an increase in the mean DEM in this temperature range by an order of magnitude, 5 hours prior to the first CME. This was associated with a gradual rise and heating of the flux rope as manifested by blue-shifts and increased non-thermal velocities in \ion{Ca}{XV}~200.97\AA, respectively. An overall upward motion of the flux ropes was measured (relative blue-shifts of $\sim12$~\kms). The measured electron density was found to be $4\times 10^9 - 2 \times10^{10}$ cm$^{-3}$ (using the ratio of \ion{Ca}{XV}~181.90\AA\ over \ion{Ca}{XV}~200.97\AA). We compare our findings with other works on the same AR to provide a unified picture of its evolution.

[20]  arXiv:1602.03688 [pdf, ps, other]
Title: Gravitational Redshift of Emission Lines in the AGN Spectra
Comments: 10 pages, 2 figures, 3 tables
Journal-ref: 2015, Astrophysics and Space Science, Volume 360, article id.7, 8 pp
Subjects: Astrophysics of Galaxies (astro-ph.GA)

The detection of gravitationally redshifted optical emission lines has been reported just for a few active galaxies. In this paper we give a short overview of studies that analyzed or exploited the detection of the gravitational redshift in optical AGN spectra. In addition, we tested the consistency of gravitational redshift as the physical origin of the redward shifts observed in their spectra using a sample of $\approx$ 50 Hamburg-ESO intermediate to high redshift quasars that are among the most luminous quasars known ($10^{47} \lesssim L \lesssim 10^{48}$ erg/s), and are expected to host very massive black holes. To this aim we modeled the line profile with accretion disk models around a black hole.

[21]  arXiv:1602.03694 [pdf, other]
Title: Search for extragalactic astrophysical counterparts of IceCube neutrino events
Comments: Appeared in the 34th International Cosmic Ray Conference Proceedings (ICRC 2015, The Hague, The Netherlands); 7 pages, 7 figures, 1 table
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

Detection of 54 very high-energy (VHE) neutrinos by the IceCube Neutrino Observatory has opened a new chapter in multi-messenger astronomy. However due to large errors in measuring the directions of the neutrino shower-type events, which dominate the current event list, it is difficult to identify their astrophysical sources. We perform cross-correlation study of IceCube neutrino events with extragalactic candidate sources using X-ray and gamma-ray selected source catalogues such as Swift-BAT, 3LAC and TeV-Cat. We apply different cuts on the X-ray and gamma-ray fluxes of the sources in these catalogs, and use different source classes in order to study correlation. We use invariant statistic and Monte Carlo simulations to evaluate statistical significance of any correlation.

[22]  arXiv:1602.03695 [pdf, ps, other]
Title: Local bursts model of primordial CMB temperature fluctuations: scattering in hydrogen lines
Comments: 20 pages, 8 figures
Journal-ref: Astron. Lett., 41, 537 (2015)
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO)

Within the framework of a flat cosmological model a propagation of an instantaneous burst of isotropic radiation is considered from the moment of its beginning at some initial redshift z0 to the moment of its registration now (at z=0). Thomson scattering by free electrons and scattering in primordial hydrogen lines Ha, Hb, Pa and Pb are considered as the sources of opacity and when calculating an albedo of a single scattering in the lines we take into account deactivation of the upper levels of transitions by background blackbody radiation. Profiles for these lines in a burst spectrum are calculated for different distances from the center of the burst and different values of z0. In the first approximation these profiles do not depend on spectrum and intensity of a burst radiation. It is shown that lines are in absorption at sufficiently large distance but emission components may appear as a distance decreases and it becomes stronger while absorption component weakens with a further distance decrease. For the sum of Ha and Hb lines the depth of absorption can reach 2e-4 while for the sum of Pa and Pb lines the maximum absorption is about 7e-6. So that the relative magnitude of temperature fluctuations lies between 1e-7 and 1e-9. The calculations were fulfiled for bursts with different initial sizes. For the same z0 the profiles of hydrogen lines are practically coinside for burst sizes lower than someone and for greater ones the lines weaken as the burst size grows.

[23]  arXiv:1602.03696 [pdf, ps, other]
Title: Supersaturation and Activity-Rotation Relation in PMS stars: the case of the Young Cluster h Per
Comments: 36 pages (the paper is 14 pages; two tables to be published online are 22 pages), 13 figures, accepted for publication in A&A
Subjects: Solar and Stellar Astrophysics (astro-ph.SR)

The magnetic activity of late-type MS stars is characterized by different regimes, and their activity levels are well described by Ro, the ratio between P_rot and the convective turnover time. Very young PMS stars show, similarly to MS stars, intense magnetic activity. However they do not show clear activity-rotation trends, and it still debated which stellar parameters determine their magnetic activity levels. To bridge the gap between MS and PMS stars, we studied the activity-rotation relation in the young cluster h Per, a ~13 Myr old cluster, that contains both fast and slow rotators, whose members have ended their accretion phase and have already developed a radiative core. It offers us the opportunity to study the activity level of intermediate-age PMS stars with different rotational velocities, excluding any interactions with the circumstellar environment. We constrained the magnetic activity levels of h Per members measuring their X-ray emission from a Chandra observation, while P_rot were obtained by Moraux et al. (2013). We collected a final catalog of 414 h Per members with known P_rot, T_eff, M_star, with 169 of them having also detected X-ray emission. We found that h Per members, with 1.0 M_sun < M_star < 1.4 M_sun, display different activity regimes: fast rotators show supersaturation, while slower rotators have activity levels compatible to the non-saturated regime. At 13 Myr h Per is therefore the youngest cluster showing activity-rotation regimes analogous to that of MS stars, indicating that, at this age, magnetic field production is likely regulated by the alpha-Omega type dynamo. Moreover we observed that supersaturation is better described by P_rot than Ro, and that the observed patterns are compatible with the hypothesis of centrifugal stripping. In this scenario we inferred that coronae can produce structures as large as ~2 R_star above the stellar surface.

[24]  arXiv:1602.03756 [pdf, other]
Title: An Optical Analysis of the Merging Cluster Abell 3888
Comments: 17 pages, 17 figures, Accepted to MNRAS 11 Feb 2016
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); Astrophysics of Galaxies (astro-ph.GA)

In this paper we present new AAOmega spectroscopy of 254 galaxies within a 30' radius around Abell 3888. We combine these data with the existing redshifts measured in a one degree radius around the cluster and performed a substructure analysis. We confirm 71 member galaxies within the core of A3888 and determine a new average redshift and velocity dispersion for the cluster of 0.1535 +\- 0.0009 and 1181 +\- 197 km/s, respectively. The cluster is elongated along an East-West axis and we find the core is bimodal along this axis with two sub-groups of 26 and 41 members detected. Our results suggest that A3888 is a merging system putting to rest the previous conjecture about the morphological status of the cluster derived from X-ray observations. In addition to the results on A3888 we also present six newly detected galaxy over-densities in the field, three of which we classify as new galaxy clusters.

[25]  arXiv:1602.03781 [pdf, ps, other]
Title: A review of the discovery reach of directional Dark Matter detection
Comments: 57 pages, 23 figures
Subjects: Cosmology and Nongalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); Instrumentation and Detectors (physics.ins-det)

Cosmological observations indicate that most of the matter in the Universe is Dark Matter. Dark Matter in the form of Weakly Interacting Massive Particles (WIMPs) can be detected directly, via its elastic scattering off target nuclei. Most current direct detection experiments only measure the energy of the recoiling nuclei. However, directional detection experiments are sensitive to the direction of the nuclear recoil as well. Due to the Sun's motion with respect to the Galactic rest frame, the directional recoil rate has a dipole feature, peaking around the direction of the Solar motion. This provides a powerful tool for demonstrating the Galactic origin of nuclear recoils and hence unambiguously detecting Dark Matter. Furthermore, the directional recoil distribution depends on the WIMP mass, scattering cross section and local velocity distribution. Therefore, with a large number of recoil events it will be possible to study the physics of Dark Matter in terms of particle and astrophysical properties. We review the potential of directional detectors for detecting and characterizing WIMPs.

[26]  arXiv:1602.03790 [pdf, ps, other]
Title: BPASS predictions for Binary Black-Hole Mergers
Authors: J. J. Eldridge (University of Auckland, NZ), E. R. Stanway (University of Warwick, UK)
Comments: 5 pages. Submitted to MNRAS
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics of Galaxies (astro-ph.GA); Solar and Stellar Astrophysics (astro-ph.SR)

Using the Binary Population and Spectral Synthesis code BPASS, we have calculated the rates, timescales and mass distributions for binary black hole mergers as a function of metallicity. We consider these in the context of the recently reported 1st LIGO event detection. We find that the event has a low probability of arising from a stellar population with initial metallicity mass fraction above $Z=0.010$. Binary black hole merger events with the reported masses are most likely in populations between Z=0.0001 and 0.002 (Z < 0.1Z_sun). The masses inferred for the black holes in the binary progenitor of GW 150914 are close to the predicted peak in the mass distribution for such events. We discuss the implications of our analysis for the electromagnetic follow-up of future LIGO event detections.

[27]  arXiv:1602.03804 [pdf, other]
Title: The Shock Dynamics of Heterogeneous YSO Jets: 3-D Simulations Meet Multi-Epoch Observations
Comments: 18 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Solar and Stellar Astrophysics (astro-ph.SR)

High resolution observations of Young Stellar Object (YSO) jets show them to be composed of many small-scale knots or clumps. In this paper we report results of 3-D numerical simulations designed to study how such clumps interact and create morphologies and kinematic patterns seen in emission line observations. Our simulations focus on clump scale dynamics by imposing velocity differences between spherical, over-dense regions which then lead to the formation of bow shocks as faster clumps overtake slower material. We show that much of the spatial structure apparent in emission line images of jets arises from the dynamics and interactions of these bow shocks. Our simulations show a variety of time-dependent features, including bright knots associated with Mach stems where the shocks intersect, a "frothy" emission structure that arises from the presence of the Non-linear Thin Shell Instability (NTSI) along the surfaces of the bow shocks, and the merging and fragmentation of clumps. Our simulations use a new non-equilibrium cooling method to produce synthetic emission maps in H$\alpha$ and [S II]. These are directly compared with multi-epoch Hubble Space Telescope (HST) observations of Herbig-Haro (HH) jets. We find excellent agreement between features seen in the simulations and the observations in terms of both proper motion and morphologies. Thus we conclude that YSO jets may be dominated by heterogeneous structures and that interactions between these structures and the shocks they produce can account for many details of YSO jet evolution.

[28]  arXiv:1602.03817 [pdf, other]
Title: Metal diffusion in smoothed particle hydrodynamics simulations of dwarf galaxies
Comments: Submitted to ApJ
Subjects: Astrophysics of Galaxies (astro-ph.GA)

We perform a series of smoothed particle hydrodynamics simulations of isolated dwarf galaxies to compare different metal mixing models. In particular, we examine the role of diffusion in the production of enriched outflows, and in determining the metallicity distributions of gas and stars. We investigate different diffusion strengths, by changing the pre-factor of the diffusion coefficient, by varying how the diffusion coefficient is calculated from the local velocity distribution, and by varying whether the speed of sound is included as a velocity term. Stronger diffusion produces a tighter [O/Fe]-[Fe/H] distribution in the gas, and cuts off the gas metallicity distribution function at lower metallicities. Diffusion suppresses the formation of low-metallicity stars, even with weak diffusion, and also strips metals from enriched outflows. This produces a remarkably tight correlation between "metal mass-loading" (mean metal outflow rate divided by mean metal production rate) and the strength of diffusion, even when the diffusion coefficient is calculated in different ways. The effectiveness of outflows at removing metals from dwarf galaxies and the metal distribution of the gas is thus dependent on the strength of diffusion. By contrast, we show that the metallicities of stars are not strongly dependent on the strength of diffusion, provided that some diffusion is present.

[29]  arXiv:1602.03821 [pdf, ps, other]
Title: Analysis of Flows Inside Quiescent Prominences as Captured by Hinode/Solar Optical Telescope
Comments: 55 pages, 13 figures, 5 tables, Accepted and Published in ApJ on Feb. 10, 2016, The Astrophysical Journal 2016
Subjects: Solar and Stellar Astrophysics (astro-ph.SR); Fluid Dynamics (physics.flu-dyn); Plasma Physics (physics.plasm-ph)

Developing an understanding of how magnetic fields can become entangled in a prominence is important for predicting a possible eruption. This work investigates the kinetic energy and vorticity associated with plasma motion residing inside quiescent prominences (QPs). These plasma flow characteristics can be utilized to improve our understanding of how the prominence maintains a stable magnetic field configuration. Three different contrast-enhanced solar prominence observations from Hinode/Solar Optical Telescope were used to construct velocity maps -- in the plane of the sky -- via a Fourier local correlation tracking program. The resulting velocities were then used to perform the first ever analysis of the two-dimensional kinetic energy and enstrophy spectra of a prominence. Enstrophy is introduced here as a means of quantifying the vorticity that has been observed in many QPs. The kinetic energy power spectral density (PSD) produced indices ranging from -1.00 to -1.60. There was a consistent anisotropy in the kinetic energy spectrum of all three prominences examined. Examination of the intensity PSD reveals that a different scaling relationship exists between the observed prominence structure and velocity maps. All of the prominences exhibited an inertial range of at least $0.8 \leq k\leq 2.0\; \textrm{rads} \: \textrm{Mm}^{-1}$. Quasi-periodic oscillations were also detected in the centroid of the velocity distributions for one prominence. Additionally, a lower limit was placed on the kinetic energy density ($\epsilon \, \sim 0.22-7.04\: \mathrm{km}^{2}\textrm{s}^{-2}$) and enstrophy density ($\omega \, \sim 1.43-13.69\: \times 10^{-16} \, \textrm{s}^{-2}$) associated with each prominence.

[30]  arXiv:1602.03831 [pdf, other]
Title: Rapid and Bright Stellar-mass Binary Black Hole Mergers in Active Galactic Nuclei
Comments: 8 pages, 2 figures
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Astrophysics of Galaxies (astro-ph.GA); General Relativity and Quantum Cosmology (gr-qc)

Laser Interferometer Gravitational-Wave Observatory, LIGO, found direct evidence of double black hole binaries emitting gravitational waves. Galactic nuclei are expected to harbor the densest population of stellar-mass black holes, accounting for as much as ~2% of the mass of the nuclear stellar cluster. A significant fraction (~30%) of these black holes can reside in binaries. We examine the fate of the black hole binaries in active galactic nuclei, which get trapped in the inner region of the accretion disk around the central supermassive black hole. We show that binary black holes can migrate into and then rapidly merge within the disk well within a Salpeter time. The binaries may also accrete a significant amount of gas from the disk, well above the Eddington rate. This could lead to detectable X-ray or gamma-ray emission, but would require hyper-Eddington accretion with a few % radiative efficiency, comparable to thin disks. We discuss implications for gravitational wave observations and black hole population studies.

[31]  arXiv:1602.03842 [pdf, other]
Title: The Rate of Binary Black Hole Mergers Inferred from Advanced LIGO Observations Surrounding GW150914
Comments: 16 pages, 6 figures, submitted to ApJL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc)

A transient gravitational-wave signal was identified in the twin Advanced LIGO detectors on September 14, 2015 at 09:50:45 UTC (GW150914). To assess the implications of this discovery, the detectors remained in operation with unchanged configurations over a period of 39 d around the time of the signal. A search of 16 days of simultaneous two-detector observational data found GW150914 to have a false alarm probability (FAP) of $2 \times 10^{-7}$. Parameter estimation followup on this trigger identifies its source as a binary black hole (BBH) merger with component masses $(m_1, m_2) = 36^{+5}_{-4}, 29^{+4}_{-4} \, M_\odot$ at redshift $z = 0.09^{+0.03}_{-0.04}$. Here we report on the constraints these observations place on the rate of BBH coalescences. Considering only GW150914, assuming that all BBHs in the universe have the same masses and spins as this event, imposing a false alarm threshold of 1 per 100 years, and assuming that the BBH merger rate is constant in the comoving frame, we infer a 90% credible range of $2-53 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$ (comoving frame). Incorporating all triggers that pass the search threshold while accounting for the uncertainty in the astrophysical origin of each trigger, we estimate a higher rate, ranging from $6-400 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$ depending on assumptions about the BBH mass distribution. All together, our various rate estimates fall in the conservative range $2-400 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$.

[32]  arXiv:1602.03846 [pdf, other]
Title: Astrophysical Implications of the Binary Black-Hole Merger GW150914
Comments: 17 pages, 4 figures, accepted to ApJL
Journal-ref: ApJL, 818, L22, 2016
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); General Relativity and Quantum Cosmology (gr-qc)

The discovery of the gravitational-wave source GW150914 with the Advanced LIGO detectors provides the first observational evidence for the existence of binary black-hole systems that inspiral and merge within the age of the Universe. Such black-hole mergers have been predicted in two main types of formation models, involving isolated binaries in galactic fields or dynamical interactions in young and old dense stellar environments. The measured masses robustly demonstrate that relatively "heavy" black holes ($\gtrsim 25\, M_\odot$) can form in nature. This discovery implies relatively weak massive-star winds and thus the formation of GW150914 in an environment with metallicity lower than $\sim 1/2$ of the solar value. The rate of binary black-hole mergers inferred from the observation of GW150914 is consistent with the higher end of rate predictions ($\gtrsim 1 \, \mathrm{Gpc}^{-3} \, \mathrm{yr}^{-1}$) from both types of formation models. The low measured redshift ($z \sim 0.1$) of GW150914 and the low inferred metallicity of the stellar progenitor imply either binary black-hole formation in a low-mass galaxy in the local Universe and a prompt merger, or formation at high redshift with a time delay between formation and merger of several Gyr. This discovery motivates further studies of binary-black-hole formation astrophysics. It also has implications for future detections and studies by Advanced LIGO and Advanced Virgo, and gravitational-wave detectors in space.

[33]  arXiv:1602.03868 [pdf, other]
Title: Swift follow-up of the Gravitational Wave source GW150914
Comments: 5 pages, to be submitted to MNRAS letters. 2 figures, 2 tables
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE); Cosmology and Nongalactic Astrophysics (astro-ph.CO)

The Advanced LIGO observatory recently reported the first direct detection of gravitational waves. We report on observations taken with the Swift satellite two days after the GW trigger. No new X-ray, optical, UV or hard X-ray sources were detected in our observations, which were focussed on nearby galaxies in the gravitational wave error region and we discuss the implications of this.

[34]  arXiv:1602.03879 [pdf, other]
Title: Collision-induced magnetic reconnection and a unified interpretation of polarization properties of GRBs and blazars
Comments: 7 pages, 4 figures, submitted to ApJL
Subjects: High Energy Astrophysical Phenomena (astro-ph.HE)

The jet composition and energy dissipation mechanism of Gamma-ray bursts (GRBs) and Blazars are fundamental questions which remain not fully understood. One plausible model is to interpret the $\gamma$-ray emission of GRBs and optical emission of blazars as synchrotron radiation of electrons accelerated from the collision-induced magnetic dissipation regions in Poynting-flux-dominated jets. The polarization observation is an important and independent information to test this model. Based on our recent 3D relativistic MHD simulations of collision-induced magnetic dissipation of magnetically dominated blobs, here we perform calculations of the polarization properties of the emission in the dissipation region and apply the results to model the polarization observational data of GRB prompt emission and blazar optical emission. We show that the same numerical model with different input parameters can reproduce well the observational data of both GRBs and blazars, especially the $90^{\circ}$ polarization angle (PA) change in GRB 100826A and the $180^{\circ}$ PA swing in Blazar 3C279. This supports a unified model for GRB and blazar jets, suggesting that collision-induced magnetic reconnection is a common physical mechanism to power the relativistic jet emission from events with very different black hole masses.