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 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 B 435 , R 625 , and Hα. 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 Hα-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.

New cataclysmic variables and other exotic binaries in the globular cluster 47 Tucanae*

“…clusters NGC 6752 and NGC 6397 for which two groups are distinguishable: a large group of CVs more concentrated in the cores of these clusters, and a small group at larger radii, as has been already pointed out by Cohn et al (2010) and Lugger et al (2017). We carried out a Kuiper test (which is as sensitive in the tails as at the median of the distribution; Kuiper 1960) to know which is the probability that the radial distributions of NGC 6397 and NGC 6752 are drawn from the same radial distribution as 47 Tuc.…”

“…If a star is blue but it was not detected in the Hα images and/or it does not show variability, that star is classified as a CV candidate. Cohn et al (2010) for NGC 6397 and from Lugger et al (2017) for NGC 6752. e Taken from Harris (1996Harris ( , 2010. f Taken from Bahramian et al (2013).…”

“…Studies of the X-ray sources in the core-collapsed GCs NGC 6397 and NGC 6752 show that these clusters have two populations of CVs: a bright population (with apparent magnitudes R625 21.5) that is more concentrated towards the core than a faint population (R625 > 21.5), for which the WD dominates the brightness Lugger et al 2017). When we compare the optical CMDs of these clusters with the optical CMD of the non core-collapsed cluster 47 Tuc, we find that in the last one we can not distinguish two groups of CVs as in NGC 6397 and NGC 6752 (Figure 2).…”

“…It is followed in number of identified CVs and CV candidates by the cluster ω Cen (27 systems - Cool et al 2013), NGC 6752 (16 systems - Lugger et al 2017) and NGC 6397 (15 systems - Cohn et al 2010). Characteristic parameters for these last two clusters are given in Table 1, and a list of the CVs and CV candidates found in them is given in Tables C1 and C2 of Appendix C. The approximate X-ray limiting luminosities of these clusters are few times 10 29 erg s −1 .…”

“…For 47 Tuc, we also analyzed the radial distribution of the CVs following the procedures described in Cohn et al (2010) and Lugger et al (2017) for fitting a parameterized surface-density profile to a main-sequence turnoff (MSTO) sample and subsamples of the CV population. This produces an estimate of the characteristic mass for all of the CVs taken together, and for the bright -and faint -CV subsamples.…”