X-Rays from the Globular Cluster G1: Intermediate-Mass Black Hole or Low-Mass X-Ray Binary?

Abstract

The globular cluster G1 (Mayall II) in M31 is the most massive (~10⁷ M_solar) stellar cluster in the Local Group, and it has the highest central velocity dispersion (~28 km s^-1). It has been claimed to host a central ~20,000 M_solar black hole, but these claims have been controversial. We investigate here the XMM-Newton detection of X-ray emission from G1 at a level of L_X~2×10³⁶ ergs s^-1. This emission could be the result of Bondi-Hoyle accretion of ionized cluster gas by a central black hole, or it could be produced by a conventional low-mass X-ray binary. A precise localization of the X-ray emission, which is not possible with the current XMM data, could distinguish between these possibilities. While such a measurement may be difficult, it is of sufficient potential importance to pursue.