Study of HST counterparts to Chandra X-ray sources in the Globular Cluster M71

TL;DR

This study uses HST observations to identify optical counterparts to Chandra X-ray sources in globular cluster M71, revealing various binary systems and suggesting a primordial origin for many sources due to the cluster's low core density.

Contribution

First comprehensive identification of optical counterparts to X-ray sources in M71, highlighting the presence of CVs and ABs and their likely primordial origin.

Findings

33 optical counterparts to 25 X-ray sources inside half-mass radius

Identification of 1 certain and 7 candidate CVs

Detection of 2 certain and 12 potential chromospherically active binaries

Abstract

We report on archival Hubble Space Telescope (HST) observations of the globular cluster M71 (NGC 6838). These observations, covering the core of the globular cluster, were performed by the Advanced Camera for Surveys (ACS) and the Wide Field Planetary Camera 2 (WFPC2). Inside the half-mass radius (r_h = 1.65') of M71, we find 33 candidate optical counterparts to 25 out of 29 Chandra X-ray sources while outside the half-mass radius, 6 possible optical counterparts to 4 X-ray sources are found. Based on the X-ray and optical properties of the identifications, we find 1 certain and 7 candidate cataclysmic variables (CVs). We also classify 2 and 12 X-ray sources as certain and potential chromospherically active binaries (ABs), respectively. The only star in the error circle of the known millisecond pulsar (MSP) is inconsistent with being the optical counterpart. The number of X-ray faint sources with L_x>4x10^{30} ergs/s (0.5-6.0 keV) found in M71 is higher than extrapolations from other clusters on the basis of either collision frequency or mass. Since the core density of M71 is relatively low, we suggest that those CVs and ABs are primordial in origin.