EDP Sciences
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Volume 447, Number 1, February III 2006
Page(s) 199 - 203
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20053855
A&A 447, 199-203 (2006)
DOI: 10.1051/0004-6361:20053855

Chandra observations of the globular cluster M 54

G. Ramsay and K. Wu

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 )

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 $\times$ 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.

Key words: Galaxy: globular clusters: individual: M 54 -- X-rays: binaries -- stars: binaries: general -- Galaxy: globular clusters: general

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