[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.