Chandra observations of the globular cluster M 54

Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, UK e-mail: gtbr@mssl.ucl.ac.uk

Received: 19 July 2005
Accepted: 8 October 2005

Abstract

We have carried out a Chandra observation of the globular cluster M 54. We detected 7 sources located within the half-mass radius of M 54, at a flux limit of 1.5  10^-15 erg s^-1 cm^-2 in the 0.3-8 keV energy band. The spatial distribution and the colour/spectral properties of the 7 sources suggest that they are likely to be cataclysmic variables or LMXBs in the globular cluster. M 54 shows the largest number of X-ray sources with luminosities greater than 10³² erg s^-1 compared to other globular clusters observed using Chandra and XMM-Newton. We searched for a correlation between the number of sources above this luminosity level with globular cluster parameters. We found evidence that the number of sources peaks at a King concentration parameter c ~ 1.7–1.9, with globular clusters which are core-collapsed or have low-c values having a smaller number of sources. We speculate on possible reasons for this.