IDENTIFICATION OF FAINTCHANDRAX-RAY SOURCES IN THE CORE-COLLAPSED GLOBULAR CLUSTER NGC 6397: EVIDENCE FOR A BIMODAL CATACLYSMIC VARIABLE POPULATION

Abstract

We have searched for optical identifications for 79 Chandra X-ray sources that lie within the halfmass radius of the nearby, core-collapsed globular cluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for Surveys Wide Field Channel imaging in Hα, R, and B. Photometry of these images allows us to classify candidate counterparts based on color-magnitude diagram location. In addition to recovering nine previously detected cataclysmic variables (CVs), we have identified six additional faint CV candidates, a total of 42 active binaries (ABs), two millisecond pulsars (MSPs), one candidate active galactic nucleus, and one candidate interacting galaxy pair. Of the 79 sources, 69 have a plausible optical counterpart.The 15 likely and possible CVs in NGC 6397 mostly fall into two groups: a brighter group of six for which the optical emission is dominated by contributions from the secondary and accretion disk, and a fainter group of seven for which the white dwarf dominates the optical emission. There are two possible transitional objects that lie between these groups. The faintest CVs likely lie near the minimum of the CV period distribution, where an accumulation is expected. The spatial distribution of the brighter CVs is much more centrally concentrated than those of the fainter CVs and the active binaries. This may represent the result of an evolutionary process in which CVs are produced by dynamical interactions, such as exchange reactions, near the cluster center and are scattered to larger orbital radii, over their lifetimes, as they age and become fainter.

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).…”

“…On the other hand, the non core-collapsed Galactic clusters show a clear profile flattening in the central few arcseconds to arcminutes and are well fitted with a King model (King 1966). Cohn et al (2010) have found that in the core-collapsed cluster NGC 6397, the population of CVs is divided in two groups, with one group of CVs brighter than the other one, and more centrally concentrated. This raises the question whether CVs in non core-collapsed clusters are also divided in two groups or not, or whether the radial and brightness distribution of CVs as seen in NGC 6397 is a signature that is typical for dynamically active clusters.…”

“…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.…”