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

TL;DR

This study uses Chandra and HST data to identify and analyze X-ray sources in globular cluster M12, revealing candidate binaries and background objects, and supporting the primordial origin of such sources.

Contribution

First detailed multi-wavelength analysis of X-ray sources in M12, linking their origins to primordial binary populations in globular clusters.

Findings

Detected 6 X-ray sources within M12's half-mass radius

Identified optical counterparts for five X-ray sources

Results support primordial origin of X-ray sources in globular clusters

Abstract

We study a Chandra X-ray Observatory ACIS-S observation of the Galactic globular clusterM12. With a 26 ks exposure time, we detect 6 X-ray sources inside the half-mass radius (2.16 arcminutes) of which two are inside the core radius (0.72 arcminutes) of the cluster. If we assume these sources are all associated with globular cluster M12, the luminosity L_{X} among these sources between 0.3-7.0 keV varies roughly from 10^{30} to 10^{32} ergs 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 2 X-ray source 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.