Journal
ASTROPHYSICAL JOURNAL
Volume 609, Issue 2, Pages 755-765Publisher
IOP PUBLISHING LTD
DOI: 10.1086/421259
Keywords
binaries : close; globular clusters : general; globular clusters : individual (M4, NGC 6121); novae, cataclysmic variables; X-rays : stars
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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 L-X approximate to 10(29) ergs s(-1) for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify six 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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