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

The globular cluster G1 (Mayall II) in M31 is the most massive (∼10M⊙) stellar cluster in the Local Group, and it has the highest central velocity dispersion (∼28 kms−1). It has been claimed to host a central ∼20,000 M⊙ black hole, but these claims have been controversial. We report here the XMM-Newton detection of X-ray emission from G1 at a level of Lx ≈ 2×1036 ergs−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. Subject headings: globular clusters: individual (Mayall II = G1) — X-rays: binaries