Merger delay time distribution of extended emission short GRBs [HEAP]

The most popular progenitor model for short duration Gamma-Ray bursts (sGRBs) is the merger of two compact objects. However, the short GRB population exhibit a certain diversity: some bursts display an extended emission (EE), continuing in soft $\gamma$-rays for a few hundreds of seconds post the initial short pulse. It is currently unclear whether the origin of such bursts is linked to compact object mergers.
Within the merger hypothesis, the redshift ($z$) distribution of short GRBs is influenced by the merger delay time, i.e., time elapsed between the merger and the formation of the binary star system, which is dominated by the time-scale for gravitational wave losses during the compact binary phase. We examine redshift distributions of short GRBs with extended emission to see whether their formation channel requires considerable delay post the star formation episode. Our results show that the $z$ distribution of EE bursts is consistent with the merger model. We attempted to compare the delay time distribution of the EE and the non-EE short bursts. However, no statistically significant difference could be seen within the limited sample size.

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N. Anand, M. Shahid and L. Resmi
Mon, 16 Oct 17

Comments: 9 pages, 10 figures, submitted to MNRAS

Agnostic Stacking of Intergalactic Doublet Absorption: Measuring the NeVIII Population [CEA]

We present a blind search for doublet intergalactic metal absorption with a method we dub `agnostic stacking’. Using forward-modelling we combine it with direct detections in the literature to measure the overall metal population. Here we apply this novel approach to the search for NeVIII in 26 high-quality COS spectra of QSOs at z>0.7. We probe an unprecedented low limit of log N>12.3 at 0.47<z<1.34 with a total pathlength ${\Delta}$z = 7.36. The method selects absorption without requiring knowledge of its source, be it observing noise, artifacts, or any line transition. Stacking this mixed population with NeVIII absorption dilutes doublet features in composite spectra in a deterministic manner. We stack potential NeVIII absorption in two regimes: absorption too weak to be statistically significant in direct line studies (12.3 < log N< 13.7), and strong absorbers (log N> 13.7). We do not detect NeVIII in either regime, and place upper limits on the population using agnostic stacking alone. Combining our measurements with direct line detections, the NeVIII population is reproduced with a single power law column density distribution of slope \b{eta} = -1.86 and normalisation log f_{13.7} = -13.99, leading to an incidence rate of strong NeVIII absorbers of dn/dz =1.38. Comparing our results with a group of 3 systems in PG1148+549, these have a 0.024% probability of arising by chance. We infer a cosmic mass density for NeVIII in the column density range 12.3 < log N < 15.0 of ${\Omega}$(NeVIII) = 2.2×10^{-8}. We translate this inferred density into an estimate of the baryon density of the NeVIII-bearing gas, and arrive at ${\Omega}b~1.8×10^{-3}$, which constitutes only 4% of the total baryonic mass. The measured NeVIII column density distribution function and cosmic density here are inconsistent with predictions of the EAGLES simulations at ${\sigma}>2.0$ significance. (abridged)

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S. Frank, M. Pieri, S. Mathur, et. al.
Mon, 16 Oct 17

Comments: 17 pages, 12 figures

The Origin of Molecular Clouds In Central Galaxies [GA]

We present an analysis of 55 central galaxies in clusters and groups with molecular gas masses and star formation rates lying between $10^{8}-10^{11}\ M_{\odot}$ and $0.5-270$ $M_{\odot}\ yr^{-1}$, respectively. We have used Chandra observations to derive profiles of total mass and various thermodynamic variables. Molecular gas is detected only when the central cooling time or entropy index of the hot atmosphere falls below $\sim$1 Gyr or $\sim$35 keV cm$^2$, respectively, at a (resolved) radius of 10 kpc. This indicates that the molecular gas condensed from hot atmospheres surrounding the central galaxies. The depletion timescale of molecular gas due to star formation approaches 1 Gyr in most systems. Yet ALMA images of roughly a half dozen systems drawn from this sample suggest the molecular gas formed recently. We explore the origins of thermally unstable cooling by evaluating whether molecular gas becomes prevalent when the minimum of the cooling to free-fall time ratio ($t_{\rm cool}/t_{\rm ff}$) falls below $\sim10$. We find: 1) molecular gas-rich systems instead lie between $10 < min(t_{\rm cool}/t_{\rm ff}) < 25$, where $t_{\rm cool}/t_{\rm ff}=25$ corresponds approximately to cooling time and entropy thresholds $t_{\rm cool} \lesssim 1$ Gyr and 35 keV~cm$^2$, respectively, 2) $min(t_{\rm cool}/t_{\rm ff}$) is uncorrelated with molecular gas mass and jet power, and 3) the narrow range $10 < min(t_{\rm cool}/t_{\rm ff}) < 25$ can be explained by an observational selection effect. These results and the absence of isentropic cores in cluster atmospheres are in tension with “precipitation” models, particularly those that assume thermal instability ensues from linear density perturbations in hot atmospheres. Some and possibly all of the molecular gas may instead have condensed from atmospheric gas lifted outward either by buoyantly-rising X-ray bubbles or merger-induced gas motions.

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F. Pulido, B. McNamara, A. Edge, et. al.
Mon, 16 Oct 17

Comments: 18 pages, 10 figures, 5 tables, submitted to ApJ

On the stability and collisions in triple stellar systems [SSA]

A significant fraction of main sequence stars are part of a triple system. We study the long-term stability and dynamical outcomes of triple stellar systems using a large number of long-term direct N-body integrations with relativistic precession. We find that the previously proposed stability criteria by Eggleton & Kiseleva 1995 and Mardling & Aarseth 2001 predict the stability against ejections reasonably well for a wide range of parameters. Assuming that the triple stellar systems follow orbital and mass distributions from FGK binary stars in the field, we find that in ~1% and ~0.5% of the triple systems lead to a direct head-on collision (impact velocity ~ escape velocity) between main sequence (MS) stars and between a MS star and a stellar-mass compact object, respectively. We conclude that triple interactions are the dominant channel for direct collisions involving a MS star in the field with a rate of one event every ~100 years in the Milky Way. We estimate that the fraction of triple systems that forms short-period binaries is up to ~23% with only up to ~13% being the result of three-body interactions with tidal dissipation, which is consistent with previous work using a secular code.

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M. He and C. Petrovich
Mon, 16 Oct 17

Comments: Submitted to MNRAS, comments welcome, 12 pages, 8 figures

Intensity Modulation of Fluorescent Line by a Finite Light Speed Effect in Accretion-Powered X-ray Pulsars [HEAP]

The X-ray line diagnostic method is a powerful tool for an investigation of plasma around accretion-powered X-ray pulsars. We point out an apparent intensity modulation of emission lines with their rotation period of neutron stars, due to the finite speed of light (we call this effect “finite light speed effect”), if the line emission mechanism is a kind of reprocessing, such as fluorescence or recombination after ionization by X-ray irradiation from pulsars. The modulation amplitude is determined by the size of the emission region, in competition with the smearing effect by the light crossing time in the emission region. This is efficient if the size of the emission region is roughly comparable to that of the rotation period multiplied by the speed of light. We apply this effect to a symbiotic X-ray pulsar, GX 1+4, a spin modulation of the intense iron line of which has been reported. The finite light speed effect can explain the observed intensity modulation, if its fluorescent region has a size of $\sim 10^{12}$ cm.

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Y. Yoshida, S. Kitamoto and A. Hoshino
Mon, 16 Oct 17

Comments: 6 pages, 5 figures

Gravitational collapse in the Schrödinger-Poisson system [CEA]

We perform a quantitative comparison between N-body simulations and the Schr\”odinger-Poisson system in 1+1 dimensions. In particular, we study halo formation with different initial conditions. We observe the convergence of various observables in the Planck constant h-bar and also test virialization. We discuss the generation of higher order cumulants of the particle distribution function which demonstrates that the Schr\”odinger-Poisson equations should not be perceived as a generalization of the dust model with quantum pressure but rather as one way of sampling the phase space of the Vlasov-Poisson system — just as N-body simulations. Finally, we quantitatively recover the scaling behavior of the halo density profile from N-body simulations.

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M. Garny and T. Konstandin
Mon, 16 Oct 17

Comments: 18 pages, 7 figures

Extinction ratios in the inner Galaxy as revealed by the VVV survey [GA]

Interstellar extinction towards the Galactic Center is large and significantly differential. Its reddening and dimming effects in red clump stars in the Galactic Bulge can be exploited to better constrain the extinction law towards the innermost Galaxy. By virtue of a deep and complete catalog of more than 30 million objets at $|l|\le2.7\deg$ and $|b|\le1.55\deg$ obtained from VVV survey observations, we apply the red clump method to infer the selective-to-total extinction ratios in the $Z$, $Y$, $J$, $H$ and $K_s$ broadband near-infrared filters. The measured values are smaller than previously reported, and are not constant, with mean values, e.g., $A_{K_S}/E(J-K_s)=0.428\pm0.005\pm0.04$ and $A_{K_S}/E(H-K_s)=1.104\pm0.022\pm0.2$. We also obtain a ratio $A_Z$:$A_Y$:$A_J$:$A_H$:$A_{K_S}$ of 7.74:5.38:3.30:1.88:1.0, implying extinction towards the Galactic Center to follow a distribution as a function of wavelength steeper than previously reported, consistent with a power law $A_{\lambda}\propto{\lambda}^{-2.47}$ in the near-infrared.

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J. Alonso-Garcia, D. Minniti, M. Catelan, et. al.
Mon, 16 Oct 17

Comments: Accepted for publication in ApJ Letters, 10 pages, 3 Figures, 2 Tables