Low Mass X-ray Binaries and Black Holes in Globular Clusters

TL;DR

This study reports the discovery of a variable stellar-mass black hole X-ray binary in a globular cluster, providing insights into black hole formation and properties in such environments, and challenging existing black hole scaling relations.

Contribution

It presents the first definitive detection of a variable black hole X-ray binary in a globular cluster, with detailed spectral analysis and implications for black hole formation theories.

Findings

Discovered a variable 4x10^39 erg/s black hole X-ray binary in a globular cluster.

Spectral evidence suggests the presence of a stellar-mass black hole with super-Eddington accretion.

The black hole's mass is below predictions from standard scaling relations for black holes.

Abstract

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.