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

We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC6121). We detect 12 X-ray sources inside the core and 19 more within the cluster halfmass radius. The limiting luminosity of this observation is Lx ≈ 10 29 erg s 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 47Tuc and NGC6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass. Subject headings: binaries: close — globular clusters: general — globular clusters: individual (M4, NGC 6121) — novae, cataclysmic variables, contact binaries — X-rays: stars