X-ray Sources and their Optical Counterparts in the Globular Cluster M4
Abstract
We report on the Chandra X-ray Observatory ACIS-S3 imaging observation of the Galactic globular cluster M4 (NGC 6121). We detect 12 X-ray sources inside the core and 19 more within the cluster half-mass radius. The limiting luminosity of this observation is Lx~10e29 erg/sec for sources associated with the cluster, the deepest X-ray observation of a globular cluster to date. We identify 6 X-ray sources with known objects and use ROSAT observations to show that the brightest X-ray source is variable. Archival data from the Hubble Space Telescope allow us to identify optical counterparts to 16 X-ray sources. Based on the X-ray and optical properties of the identifications and the information from the literature, we classify two (possibly three) sources as cataclysmic variables, one X-ray source as a millisecond pulsar and 12 sources as chromospherically active binaries. Comparison of M4 with 47 Tuc and NGC 6397 suggests a scaling of the number of active binaries in these clusters with the cluster (core) mass.
Keywords
Cite
@article{arxiv.astro-ph/0402197,
title = {X-ray Sources and their Optical Counterparts in the Globular Cluster M4},
author = {Cees Bassa and David Pooley and Lee Homer and Frank Verbunt and Bryan M. Gaensler and Walter H. G. Lewin and Scott F. Anderson and Bruce Margon and Victoria M. Kaspi and Michiel van der Klis},
journal= {arXiv preprint arXiv:astro-ph/0402197},
year = {2009}
}
Comments
11 pages, 6 figures, accepted for publication in ApJ. Figure 1 and 5 are of reduced quality