SIMBAD references

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 1.5x10^–15erg/s/cm² in the 0.3-8keV 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 10³²erg/s 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.