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 associated 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 galaxies, 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(>L_X) ∝ L^{-0.96 ± 0.11}_X. Although we do not find any statistical requirement for a cut-off luminosity of L_c∼ 10⁴⁰ erg s^-1, as has been reported previously, we are not able to rule out its presence. Also, we find that the number of ULXs per unit galaxy mass, S^u, decreases with increasing galaxy mass for ULXs associated with spiral galaxies, and is well modelled with a power law of form S^u ∝ M^{-0.64 ± 0.07}. This is in broad agreement with previous results, and is likely to be a consequence of the decrease in specific star formation and increase in metallicity with increasing spiral galaxy mass. S^u is consistent with being constant with galaxy mass for sources associated with elliptical galaxies, implying this older ULX population traces stellar mass rather than star formation.