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

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.Comment: 11 pages, 6 figures, accepted for publication in ApJ. Figure 1 and 5 are of reduced qualit