X-ray Sources and their Optical Counterparts in the Galactic Globular Cluster M12 (NGC 6218)

Abstract

We study a Chandra X-ray Observatory ACIS-S observation of the Galactic globular cluster M12. With a 26 ks exposure time, we detect six X-ray sources inside the half-mass radius (2farcm16) of which two are inside the core radius (0farcm72) of the cluster. If we assume that these sources are all associated with globular cluster M12, the luminosity L _X among these sources between 0.3 and 7.0 keV varies roughly from 10³⁰ to 10³² erg s^-1. For identification, we also analyzed the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) and Wide Field and Planetary Camera 2 (WFPC2) data and identified the optical counterparts to five X-ray sources inside the HST ACS field of view. According to the X-ray and optical features, we found 2-5 candidate active binaries (ABs) or cataclysmic variables (CVs) and 0-3 background galaxies within the HST ACS field of view. Based on the assumption that the number of X-ray sources scales with the encounter rate and the mass of the globular cluster, we expect two X-ray sources inside M12, and the expectation is consistent with our observational results. Therefore, the existence of identified X-ray sources (possible CVs or ABs) in M12 suggests the primordial origin of X-ray sources in globular clusters which is in agreement with previous studies.