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

TL;DR

This paper reports the discovery of a very faint X-ray transient in globular cluster M15, likely a neutron star low-mass X-ray binary, with detailed analysis of its X-ray and optical properties from archival data.

Contribution

First identification and detailed multi-epoch analysis of a very faint X-ray transient in M15, linking X-ray and optical observations to its nature.

Findings

Detected X-ray transient at 6×10^{33} erg/s luminosity

Identified a likely main sequence companion with UV excess

Observed behavior similar to Galactic Center very faint transients

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*10^{33} ergs/s 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*10^{31} ergs/s 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/UV counterpart with a (possibly transient) UV excess from archival HST 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.