Chandra's First Decade of Discovery

Session 10: Globular Clusters

Globular Cluster X-ray Sources

David Pooley, University of Wisconsin

Globular clusters and X-ray astronomy have a long and fruitful history. Uhuru and OSO-7 revealed a population of highly luminous (> 1036 erg/s) X-ray sources in globular clusters, and Einstein and ROSAT revealed a larger population of low luminosity (< 1033 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 ROSAT observations of 55 globular clusters, about 25 low-luminosity X-ray sources were found. To date, 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.

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Extragalactic globular clusters with Chandra

Tom Maccarone, University of Southampton
Arunav Kundu (Michigan State), Steve Zepf (Michigan State)

Globular clusters produce bright (LX>1036 ergs/sec) X-ray sources with efficiencies, on average, about 100 times as large as those of field stellar populations. Despite this efficiency, the Milky Way still contains fewer than 20 such sources, making statistical studies of which cluster properties are most important for producing X-ray sources difficult. Furthermore, the Milky Way's globular cluster population is relatively homogeneous in metallicity. There are many nearby elliptical galaxies with 10-100 times as many globular clusters as the Milky Way, and with significantly larger numbers of metal rich clusters. Chandra's excellent angular resolution has made it possible both to detect these sources against the strong gas background often found in these galaxies, and to localize these sources well enough to associate them unambiguously with optically detected globular clusters. An important and surprising result of this work is that metal rich globular clusters are significantly more likely to contain X-ray sources than metal poor clusters. The large samples of extragalactic clusters also make it possible to find rare objects. Numerous ultraluminous X-ray sources have been found in extragalactic clusters. Recent results have shown that a growing fraction of these objects can be proved to be variable, indicating that most of the emission must be coming from a single source, providing observational evidence that black holes in globular clusters are far more common than was once thought.

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Deep Chandra Studies of Millisecond Pulsars in Globular Clusters

Slavko Bogdanov, McGill University
Jonathan E. Grindlay (Harvard)

The unprecedented sub-arcsecond angular resolution of the Chandra X-ray Observatory has proven to be ideally suited for studies of globular clusters, where “recycled” millisecond pulsars are found in great abundance. Deep observations of 47 Tuc, NGC 6397, and M28, totaling over 1 Ms of combined exposure, have greatly improved our understanding of the X-ray properties of these objects. In particular, these surveys have revealed that most millisecond pulsars exhibit faint, soft thermal emission from their heated magnetic polar caps, while “black widow” and the peculiar dynamically exchanged binary millisecond pulsars exhibit synchrotron X-rays due to interaction of their relativistic winds with a close binary partner. I will present the results of our X-ray studies of the Galactic population of millisecond pulsars, which offer unique insight into binary evolution, internal globular cluster dynamics, pulsar winds, collisionless shock, and the neutron star equation of state.

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