Study of Hubble Space Telescope counterparts to Chandra X-ray sources in the globular cluster M 71

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany e-mail: rhuang@mpe.mpg.de
² Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
³ NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
⁴ Department of Physics, University of Alberta, Edmonton, Alberta, Canada
⁵ Institute of Astronomy and Astrophysics, Academia Sinica, Taipei 10617, Taiwan

Received: 28 October 2008
Accepted: 2 December 2009

Abstract

Aims. We report on archival Hubble Space Telescope (HST) observations of the globular cluster M 71 (NGC 6838).

Methods. 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 = 165) of M 71, we find 33 candidate optical counterparts to 25 out of 29 Chandra X-ray sources, while 6 possible optical counterparts to 4 X-ray sources are found outside the half-mass radius.

Results. 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 X-ray sources as certain and 12 as 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.

Conclusions. The number of X-ray faint sources with > 4×10³⁰ erg s^-1 (0.5–6.0 keV) found in M 71 is higher than extrapolations from other clusters on the basis of either collision frequency or mass. Since the core density of M 71 is relatively low, we suggest that those CVs and ABs are primordial in origin.