[Other -- Invited ]

Globular Cluster X-ray Sources

David Pooley, University of Wisconsin

Globular clusters and X-ray astronomy have a long and fruitful history. {\it Uhuru} and {\it OSO-7} revealed a population of highly luminous ($> 10\^{36}$ erg/s) X-ray sources in globular clusters, and {\it Einstein} and {\it ROSAT} revealed a larger population of low luminosity ($< 10\^{33}$ erg/s) X-ray sources. It was realized early on that the high luminosity sources were low-mass X-ray binaries in outburst and that they were orders of magnitude more abundant per unit mass in globular clusters than in the rest of the Galaxy. However, the low luminosity sources proved difficult to classify. Many ideas were put forth --- including low-mass X-ray binaries in quiescence (qLMXBs), cataclysmic variables (CVs), active main-sequence binaries (ABs), and millisecond pulsars (MSPs) --- but secure identifications were scarce. In {\it ROSAT} observations of 55 globular clusters, about 25 low-luminosity X-ray sources were found. To date, {\it Chandra} has observed over 80 Galactic globular clusters, mainly with ACIS, and these observations have revealed over 1500 X-ray sources. The superb angular resolution has allowed for many counterpart identifications, providing clues to the nature of this population. It is a heterogenous mix of qLMXBs, CVs, ABs, and MSPs, and it has been shown that the qLMXBs and CVs are both, in part, overabundant like the luminous LMXBs. The number of X-ray sources in a globular cluster correlates very well with its encounter frequency. This points to dynamical formation scenarios for the X-ray sources and shows them to be excellent tracers of the complicated internal dynamics of globular clusters. The relation between the encounter frequency and the number of X-ray sources has been used to suggest that we have misunderstood the dynamical states of globular clusters.