2XMM ultraluminous X-ray source candidates in nearby galaxies

Ultraluminous X-ray sources (ULXs) are some of the most enigmatic X-ray bright sources known to date. It is generally accepted that they cannot host black holes as large as those asso-ciated with active galaxies, but they appear to be significantly more luminous than their better understood Galactic X-ray binary (XRB) cousins, while displaying an intriguing combination of differences and similarities with them. Through studying large, representative samples of these sources we may hope to enhance our understanding of them. To this end, we derive a large catalogue of 650 X-ray detections of 470 ULX candidates, located in 238 nearby galax-ies, by cross-correlating the 2XMM Serendipitous Survey with the Third Reference Catalogue of Bright Galaxies. The presented dedicated catalogue offers a significant improvement over those previously published in terms of both the number and the contribution of background contaminants, e.g. distant quasars, which we estimate to be at most 24 per cent, but more likely ∼17 per cent. To undertake population studies, we define a ‘complete ’ sub-sample of sources compiled from observations of galaxies with sensitivity limits below 1039 erg s−1. The luminosity function of this sample is consistent with a simple power law of form N(>LX)