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

• Published in: The Astrophysical journal Vol. 705; no. 1; pp. 175 - 183
• Main Authors: Lu, Ting-Ni, Kong, Albert K. H, Bassa, Cees, Verbunt, Frank, Lewin, Walter H. G, Anderson, Scott F, Pooley, David
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 01.11.2009; IOP
• Subjects: Astronomy; ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; BINARY STARS; CAMERAS; COSMIC RAY SOURCES; COSMIC X-RAY SOURCES; Earth, ocean, space; ERUPTIVE VARIABLE STARS; Exact sciences and technology; GALAXIES; LUMINOSITY; MASS; NOVAE; OPTICAL PROPERTIES; PHYSICAL PROPERTIES; STARS; TELESCOPES; VARIABLE STARS; X-rays: binaries; globular clusters: individual (M12); Novae; Binary X ray source; Cosmic x-ray sources; binaries: close; Globular clusters; Star clusters; Galactic cluster; Optical counterpart; novae, cataclysmic variables; X-ray binary stars; Close binary stars; Cataclysmic variable stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/705/1/175

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 (216) of which two are inside the core radius (072) 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.