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Register a new userCould Black-Hole X-Ray Binaries Be Detected in Globular Clusters?
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We consider the implications of the presence of ~1 stellar-mass black hole (BH) at the center of a dense globular cluster. We show that BH X-ray binaries formed through exchange interactions are likely to have extremely low duty cycles (<0.001), consistent with the absence of BH X-ray transients in Galactic globular clusters. In contrast, we find that BH X-ray binaries formed through tidal capture would be persistent, bright X-ray sources. Given the absence of any such source and the very high interaction rates, we conclude that tidal capture of a main-sequence star by a BH most likely leads to the complete disruption of the star.
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We investigate properties of black hole (BH) binaries formed in globular clusters via dynamical processes, using direct N-body simulations. We pay attention to effects of BH mass function on the total mass and mass ratio distributions of BH binaries ejected from clusters. Firstly, we consider BH populations with two different masses in order to learn basic differences from models with single-mass BHs only. Secondly, we consider continuous BH mass functions adapted from recent studies on massive star evolution in a low metallicity environment, where globular clusters are formed. In this work, we consider only binaries that are formed by three-body processes and ignore stellar evolution and primordial binaries for simplicity. Our results imply that most BH binary mergers take place after they get ejected from the cluster. Also, mass ratios of dynamically formed binaries should be close to one or likely to be less than 2:1. Since the binary formation efficiency is larger for higher-mass BHs, it is likely that a BH mass function sampled by gravitational-wave observations would be weighed toward higher masses than the mass function of single BHs for a dynamically formed population. Applying conservative assumptions regarding globular cluster populations such as small BH mass fraction and no primordial binaries, the merger rate of BH binaries originated from globular clusters is estimated to be at least 6.5 per yr per Gpc^3. Actual rate can be up to more than several times of our conservative estimate.
We study the compact binary population in star clusters, focusing on binaries containing black holes, using a self-consistent Monte Carlo treatment of dynamics and full stellar evolution. We find that the black holes experience strong mass segregation and become centrally concentrated. In the core the black holes interact strongly with each other and black hole-black hole binaries are formed very efficiently. The strong interactions, however, also destroy or eject the black hole-black hole binaries. We find no black hole-black hole mergers within our simulations but produce many hard escapers that will merge in the galactic field within a Hubble time. We also find several highly eccentric black hole-black hole binaries that are potential LISA sources, suggesting that star clusters are interesting targets for space-based detectors. We conclude that star clusters must be taken into account when predicting compact binary population statistics.
Studies of nearby galaxies reveal that roughly half of their low mass X-ray binary (LMXB) populations are associated with globular clusters (GCs). We have established that the LMXB hosting frequency is correlated to various GC properties such as mass and metallicity. While the X-ray luminosities of a few of the brightest LMXBs in GCs are consistent with the accreting object being a black hole (BH), the only definitive way to distinguish between a black hole and multiple superposed sources in a GC is to detect variability. We have discovered just such a variable 4x10^39 erg/s black hole X-ray binary in a low metallicity globular cluster in the halo of NGC 4472. The change in the X-ray spectrum between the bright and faint epochs suggests that the luminosity variation is due to eclipsing by a warped accretion disk. The optical spectrum of this source also reveals strong, broad, [O III] lambda 5007 and [O III] lambda 4959 emission. An analysis of the X-ray spectrum suggests that the [O III] lines are produced by the photoionization of a wind driven by a stellar mass black hole accreting mass at or above its Eddington luminosity. As it is dynamically implausible to form an accreting stellar mass BH system in a GC with an intermediate mass BH it appears that this massive globular cluster does not harbor an intermediate mass BH. The inferred mass of this BH falls well below the extrapolation of the well known M_BH-sigma and M_BH-M_Stellar relations to this GC. Therefore our analysis suggests that not all old, metal poor stellar systems form black holes consistent with these relations, which have been established for much more massive stellar systems.
The X-ray emission from normal elliptical galaxies has two major components: soft emission from diffuse gas and harder emission from populations of accreting (low-mass) stellar X-ray binaries (LMXB). If LMXB populations are tied to the field stellar populations in galaxies, their total X-ray luminosities should be proportional to the optical luminosities of galaxies. However, recent ASCA and Chandra X-ray observations show that the global luminosities of LMXB components in ellipticals exhibit significant scatter at a given optical luminosity. This scatter may reflect a range of evolutionary stages among LMXB populations in ellipticals of different ages. If so, the ratio of the global LMXB X-ray luminosity to the galactic optical luminosity, L_LMXB/L_opt, may be used to determine when the bulk of stars were formed in individual ellipticals. To test this, we compare variations in L_LMXB/L_opt for LMXB populations in ellipticals to optically-derived estimates of stellar ages in the same galaxies. We find no correlation, implying that L_LMXB/L_opt variations are not good age indicators for ellipticals. Alternatively, LMXBs may be formed primarily in globular clusters (through stellar tidal interactions), rather than in the stellar fields of galaxies. Since elliptical galaxies exhibit a wide range of globular cluster populations for a given galaxian luminosity, this may induce a dispersion in the LMXB populations of ellipticals with similar optical luminosities. Indeed, we find that L_LMXB/L_opt ratios for LMXB populations are strongly correlated with the specific globular cluster frequencies in elliptical galaxies. This suggests that most LMXBs were formed in globular clusters.
We present results of Hubble Space Telescope and Chandra X-ray Observatory observations of globular clusters (GCs) and low-mass X-ray binaries (LMXBs) in the central regions of Centaurus A. Out of 440 GC candidates we find that 41 host X-ray point sources that are most likely LMXBs. We fit King models to our GC candidates in order to measure their structural parameters. We find that GCs that host LMXBs are denser and more compact, and have higher encounter rates and concentrations than the GC population as a whole. We show that the higher concentrations and masses are a consequence of the dependence of LMXB incidence on central density and size plus the general trend for denser GCs to have higher masses and concentrations. We conclude that neither concentration nor mass are fundamental variables in determining the presence of LMXBs in GCs, and that the more fundamental parameters relate to central density and size.
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