Exploiting a new catalogue of ultraluminous X-ray sources

Abstract

Ultraluminous X-ray sources (ULXs) are objects with extreme X-ray luminosities ( 10ergs) outside galaxy centres. They are now thought to be a heterogenous class of astronomical object, with individual sources
powered either by sub-Eddington accretion onto intermediate mass black holes or super-Eddington
accretion onto stellar mass black holes and neutron stars.
We build and present a large, cleaned, multi-mission catalogue of 1843 ULX candidates in 951 different host galaxies.
Comparing with former, concurrent and subsequent ULX catalogues, it is the largest ULX catalogue ever compiled at the time of publication. While some catalogues published since are larger, our candidates are more likely to be reliable due to our use of stricter constraints during assembly.
Having utilised data from three distinct X-ray missions, we reveal 689 new ULX candidates, one of which we present in detail and 71 HLX candidates which we explore as a sub-population of the ULX class.
Further cleaning to remove our HLX sub-sample of interlopers leaves 37 good HLX candidates, which we then whittle down to the 20 best candidates. We compare these to two well-established members of the HLX class, an intermediate mass black hole and a pulsating source, as potential archetypes of the wider population. Our findings instead indicate that they are perhaps at the extremeties of a highly heterogeneous class and while some of our HLX candidates are found to be comparable, the majority of our sources fall somewhere in between or carve out their own portion of parameter space.
Finally, we present the findings from two successful observing proposals targeting a ULX candidate identified both within our catalogue and the all-sky surveys from the mission. We explore the source's validity as a ULX candidate, search for the presence of a neutron star accretor and the possible existence of a surrounding bubble nebula.