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arXiv · astro-ph/0402197

X-ray Sources and their Optical Counterparts in the Globular Cluster M4

Cees Bassa·David Pooley·Lee Homer·Frank Verbunt·Bryan M. Gaensler·Walter H. G. Lewin·Scott F. Anderson·Bruce Margon·Victoria M. Kaspi·Michiel van der Klis
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Abstract

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.

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Cees Bassa, David Pooley, Lee Homer, Frank Verbunt, Bryan M. Gaensler, Walter H. G. Lewin, Scott F. Anderson, Bruce Margon, Victoria M. Kaspi, Michiel van der Klis. 2004-03-22. X-ray Sources and their Optical Counterparts in the Globular Cluster M4. https://doi.org/10.1086/421259

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