A New, Clean Catalogue of Extragalactic Non-nuclear X-ray Sources

Abstract

We use the XMM-Newton Serendipitous Source Catalogue to create a catalogue of non-nuclear extragalactic X-ray sources that is both large, containing 1,464 sources, and clean, with steps taken to minimise contamination. This catalogue contains 372 ultraluminous X-ray sources (ULXs), objects with extreme luminosities (ergs) higher than expected for a stellar-mass black hole (BH) accreting at its Eddington limit, and thought to mainly be super-Eddington accreting compact objects. We find the colour properties of ULXs to be similar to lower-luminosity sources, distinct from active galactic nuclei, and similar between populations in spiral and elliptical galaxies. Our catalogue contains eight new candidate intermediate-mass BHs (IMBHs) amongst the most luminous sources, 33 transient ULXs, and five ULXs with high short-term variability. Of these highly variable ULXs, we closely examine M51 ULX-7, which exhibits a persistently hard spectrum and evidence of band-limited noise, arguments in favour of it being an IMBH. However, a possible high-energy spectral turnover and its high dynamic range of flux bear similarities to properties observed in neutron star ULXs discovered to date. A useful subpopulation of our catalogue are sources at the Eddington Threshold (ergs), the luminosity regime in which stellar-mass BHs may be accreting at or just below the Eddington limit. Our catalogue contains 666 such objects, four of which we studied in detail and found to demonstrate the heterogeneous nature of this luminosity regime, including high-luminosity sub-Eddington accretion states, the ultraluminous supersoft accretion regime, and sources in intermediate accretion modes. Finally, we searched for evidence of the most highly obscured and down-scattered super-Eddington accreting sources by looking for ultraluminous ultraviolet (ULUV) sources in UV source catalogues. We discovered six ULUV candidates in the Swift UVOT catalogue that are ideal for follow-up observations using HST.