X-ray Sources in Galactic Globular Clusters

Abstract

I review recent work on X-ray sources in Galactic globular clusters, identified with low-mass X-ray binaries (LMXBs), cataclysmic variables (CVs), millisecond pulsars (MSPs) and coronally active binaries by Chandra. Faint transient LMXBs have been identified in several clusters, challenging our understanding of accretion disk instabilities. Spectral fitting of X-rays from quiescent LMXBs offers the potential to constrain the interior structure of neutron stars. The numbers of quiescent LMXBs scale with the dynamical interaction rates of their host clusters, indicating their dynamical formation. Large numbers of CVs have been discovered, including a very faint population in NGC 6397 that may be at or beyond the CV period minimum. Most CVs in dense clusters seem to be formed in dynamical interactions, but there is evidence that some are primordial binaries. Radio millisecond pulsars show thermal X-rays from their polar caps, and often nonthermal X-rays, either from magnetospheric emission, or from a shock between the pulsar wind and material still flowing from the companion. Chromospherically active binaries comprise the largest number of X-ray sources in globular clusters, and their numbers generally scale with cluster mass, but their numbers seem to be reduced in all globular clusters compared to other old stellar populations.