The Discovery of a Very Faint X-Ray Transient in the Globular Cluster M15

Abstract

We have identified an X-ray transient (hereafter M15 X-3) in the globular cluster M15 from an archival Chandra grating observation. M15 X-3 appears at an X-ray luminosity of 6 × 10³³ erg s^-1 with a spectrum consistent with an absorbed power law of photon index 1.51 ± 0.14. The object is identifiable in archival Chandra HRC-I observations with an X-ray luminosity of (2-6)×10³¹ erg s^-1 and apparently soft colors, suggesting a neutron star low-mass X-ray binary in quiescence. We also observe it in outburst in a 2007 Chandra HRC-I observation, and in archival 1994-1995 ROSAT HRI observations. We identify a likely optical/ultraviolet (UV) counterpart with a (possibly transient) UV excess from archival Hubble Space Telescope data, which suggests a main-sequence companion. We argue that M15 X-3's behavior is similar to that of the very faint X-ray transients which have been observed in the Galactic center. We discuss several explanations for its very low X-ray luminosity, with the assumption that we have detected its companion. M15 X-3's uniquely low extinction and well determined distance make it an excellent target for future studies.