Identification of Faint Chandra X-ray Sources in the Core-Collapsed Globular Cluster NGC 6752

Abstract

We have searched for optical identifications for 39 Chandra X-ray sources that lie within the 1.9 arcmin half-mass radius of the nearby (d = 4.0 kpc), core-collapsed globular cluster, NGC 6752, using deep Hubble Space Telescope ACS/WFC imaging in B435, R625, and H alpha. Photometry of these images allows us to classify candidate counterparts based primarily on color-magnitude and color-color diagram location. The color-color diagram is particularly useful for quantifying the H alpha line equivalent width. In addition to recovering 11 previously detected optical counterparts, we propose 20 new optical IDs. In total, there are 16 likely or less certain cataclysmic variables (CVs), nine likely or less certain chromospherically active binaries, three galaxies, and three active galactic nuclei (AGNs). The latter three sources, which had been identified as likely CVs by previous investigations, now appear to be extragalactic objects based on their proper motions. As we previously found for NGC 6397, the CV candidates in NGC 6752 fall into a bright group that is centrally concentrated relative to the turnoff-mass stars and a faint group that has a spatial distribution that is more similar to that of the turnoff-mass stars. This is consistent with an evolutionary scenario in which CVs are produced by dynamical interactions near the cluster center and diffuse to larger radius orbits as they age.Comment: 26 pages, 18 figure