Ultraluminous X-ray Sources: The Extremes of Accretion

Ultraluminous X-ray Sources (ULXs) represent the most luminous extremes of the accreting binary population, systems in which a compact object is feeding on material provided by its companion star. Following a series of remarkable recent discoveries, we now know that some of the most luminous ULXs are actually powered by pulsars (i.e. neutron stars) accreting far above their Eddington limits (the point at which the inward gravitational attraction should be balanced by outward radiation pressure, resulting in a maximum predicted luminosity).

The most extreme case among these ULX pulsars (ULXPs) exceeds its Eddington limit by a factor of ~500! These systems potentially offer a rare opportunity to understand the super-Eddington regime that may be required to grow the supermassive black holes (SMBHs) now being observed when the Universe was still in its infancy.

However, these potential connections are complicated by the fact that we do not understand the makeup of the ULX population. Although there are indications that neutron stars similar to ULXPs could potentially dominate the population, to date there are only six ULXs that have been robustly identified as neutron stars. Furthermore, we do not yet understand how these ULXPs are able to reach such extreme luminosities and accretion rates, and in particular whether extreme magnetic fields are required to do so.

Key to learning about these issues - the make-up of the ULX population and the physics of accretion at extreme rates - is to grow the sample of known ULXPs. We have recently completed efforts to combine new X-ray datasets from Chandra, Swift and XMM-Newton to produce the largest sample of ULXs to date (Walton et al. 2022), ~tripling the number of ULXs in comparison to the older ULX catalogues available.

In addition, data from the eROSITA survey will soon be available, with which we can further expand our ULX catalogue. With suitable follow-up of key targets from these efforts to find new ULXs, we therefore expect to be able to significantly expand the known population of ULX pulsars and place meaningful constraints on their contribution to the broader ULX population.

This new sample will also enable us to understand the nature of the accretion (and outflows) in these remarkable systems, and determine whether they really are local analogues for early-universe SMBH growth.