Ultraluminous X-ray sources in the era of eROSITA

Accretion onto stellar remnants (black holes and neutron stars), is classically restricted to the Eddington limit where radiation - produced via the conversion of potential energy to heat by infalling material - is balanced by gravity. However, super-Eddington accretion is an observational fact and results from non-spherical inflows (accretion discs) which the Eddington limit does not account for. The details of the process require us to investigate the coupling of fluid (plasma) to radiation and magnetic fields - this is a complex process that is both theoretically and computationally challenging. Observationally, stellar remnants accreting at such rates account for the population of so-called ultra-luminous X-ray sources (ULXs) which form an important component of X-ray all-sky surveys. Their appearance (energy spectra and brightness) are a function of the fluid dynamics of the accretion flow, affected by tidal forces from the secondary, donor star and Lense-Thirring torques from frame-dragging (a consequence of General Relativity). At present the impact of these effects in deriving population statistics have not been considered.

Although long considered to play host to black holes, since the last X-ray all-sky survey by ROSAT, it has been discovered that some ULXs show pulsations in their X-ray lightcurves, immediately associating them with neutron star accretors. Such pulsating ULXs (PULXs) challenge our understanding of accretion even more so (as for the same mass transfer rate from the donor star they are even more super-Eddington) but their sample size is very small - presently only 5 out of 100s of ULXs.

With the launch of eROSITA - a joint German/Russian mission launching on the Russian Spectrum-Roentgen-Gamma (SRG) satellite in early 2019 - the X-ray sky will be studied to greater depth. As external collaborator, my PhD student will have access to the wealth of new data. The project will entail developing fast algorithms to process the data, categorize new sources (based on multi-wavelength cross-matching where possible) and discover new PULXs using advanced timing techniques. Based on cross-mission archival data, the student will also extract historical lightcurves and perform statistical inference techniques to predict the role of precession (the effect of tidal and Lense-Thirring torque) and make comparisons to the theoretical distribution from stellar population synthesis codes. This will be cross-checked across the expanded sample from eROSITA.