Discovery of an X-ray burst from X2127+11 in the globular cluster M15

Abstract

THE nature and evolution of binary X-ray sources in globular clusters are poorly understood. Among the ∼10 bright X-ray sources in globular clusters, X2127+11 in M15 is very important because it is the only one for which the optical counterpart, AC211, and orbital period, 8.5 h, are both known1. We have detected a type I X-ray burst from X2127+11, which was the only bright X-ray source in a globular cluster from which no burst had been detected. The burst was of long duration (≳150s) and had a precursor separated from the main peak by ∼6 s. This means that the burst was very energetic and that a large photospheric expansion occurred. The observed burst peak luminosity, well above 1038 erg s−1, is hard to reconcile with the standard idea that X2127+11 is screened from direct view by an accretion disk. This latter view arises from consideration of the low ratio of X-ray to optical luminosity, which suggests that only a small fraction of X-rays is visible through scattering by an accretion-disk corona around the source2–4.