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RE: Stellar Black Holes
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Title: The Black Hole Mass Distribution in the Galaxy
Authors: Feryal Ozel, Dimitrios Psaltis, Ramesh Narayan, Jeffrey E. McClintock
(Version v2)

We use dynamical mass measurements of 16 black holes in transient low-mass X-ray binaries to infer the stellar black hole mass distribution in the parent population. We find that the observations are best described by a narrow mass distribution at 7.8 1.2 Msolar. We identify a selection effect related to the choice of targets for optical follow-ups that results in a flux-limited sample. We demonstrate, however, that this selection effect does not introduce a bias in the observed distribution and cannot explain the absence of black holes in the 2-5 solar mass range. On the high mass end, we argue that the rapid decline in the inferred distribution may be the result of the particular evolutionary channel followed by low-mass X-ray binaries. This is consistent with the presence of high-mass black holes in the persistent, high-mass X-ray binary sources. If the paucity of low-mass black holes is caused by a sudden decrease of the supernova explosion energy with increasing progenitor mass, this would have observable implications for ongoing transient surveys that target core-collapse supernovae. Our results also have significant implications for the calculation of event rates from the coalescence of black hole binaries for gravitational wave detectors.

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Title: Stellar-mass black holes in star clusters: implications for gravitational wave radiation
Authors: Sambaran Banerjee, Holger Baumgardt, Pavel Kroupa

We study the dynamics of stellar-mass black holes (BH) in star clusters with particular attention to the formation of BH-BH binaries, which are interesting as sources of gravitational waves (GW). We examine the properties of these BH-BH binaries through direct N-body simulations of star clusters using the GPU-enabled NBODY6 code. We perform simulations of N <= 10^5 Plummer clusters of low-mass stars with an initial population of BHs. Additionally, we do several calculations of star clusters confined within a reflective boundary mimicking only the core of a massive cluster. We find that stellar-mass BHs with masses ~ 10 solar mass segregate rapidly into the cluster core and form a sub-cluster of BHs within typically 0.2 - 0.5 pc radius, which is dense enough to form BH-BH binaries through 3-body encounters. While most BH binaries are ejected from the cluster by recoils received during super-elastic encounters with the single BHs, few of them harden sufficiently so that they can merge via GW emission within the cluster. We find that for clusters with N \ga 5 x 10^4, typically 1 - 2 BH-BH mergers occur within them during the first ~ 4 Gyr of evolution. Also for each of these clusters, there are a few escaping BH binaries that can merge within a Hubble time, most of the merger times being within a few Gyr. These results indicate that intermediate-age massive clusters constitute the most important class of candidates for producing dynamical BH-BH mergers. Old globular clusters cannot contribute significantly to the present-day BH-BH merger rate since most of the mergers from them would have occurred earlier. In contrast, young massive clusters are too young to produce significant number of BH-BH mergers. Our results imply significant BH-BH merger detection rates for the proposed "Advanced LIGO" GW detector.

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Title: On The Maximum Mass of Stellar Black Holes
Authors: Krzysztof Belczynski, Tomasz Bulik, Chris L. Fryer, Ashley Ruiter, Jorick S. Vink, Jarrod R. Hurley

We present the spectrum of compact object masses: neutron stars and black holes that originate from single stars in different environments. In particular, we show the dependence of maximum black hole mass on metallicity and on some specific wind mass loss rates (e.g., Hurley et al. and Vink et al.). We demonstrate that the highest mass black holes observed in the Galaxy M_bh ~ 15 Msun in the high metallicity environment (Z=Zsun=0.02) can be explained with stellar models and the wind mass loss rates adopted here. To reach this conclusion it was required to set Luminous Blue Variable mass loss rates at the level of ~ 0.0001 Msun/yr and to employ metallicity dependent Wolf-Rayet winds. With the calibrated (on Galactic black hole mass measurements) winds the maximum black hole mass predicted for moderate metallicity (Z=0.3 Zsun=0.006) is M_bh,max = 30 Msun. This is a rather striking finding as the mass of the most massive known stellar black hole is M_bh = 23-34 Msun and, in fact, it is located in a small star forming galaxy with moderate metallicity. It is also predicted that in the very low (globular cluster-like) metallicity environment the maximum black hole mass can be as high as M_bh,max = 80 Msun (Z=0.01 Zsun=0.0002). We emphasize that our results were obtained for single stars only and that binary interactions may alter the predictions (e.g., accretion from a close companion) for maximum black hole masses. This is strictly a proof-of-principle study which demonstrates that stellar models can naturally explain even the most massive known stellar black holes.

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