Accretion onto compact objects powers some of the most luminous and extreme phenomena in the universe. Accreting supermassive black holes ultimately power all of the different populations of active galactic nuclei (AGN), while stellar remnant black holes and neutron stars power luminous X-ray binary systems (XRBs) that are scattered throughout all major galaxies. This accretion process also drives powerful outflows (winds, jets) that allow these compact sources to impact their surroundings, and are responsible for driving the 'feedback' processes that dictate the observed co-evolution of supermassive black holes and their host galaxies. Understanding this accretion process (and its by-products) is therefore of critical importance. In particular, there are still major questions to be answered regarding how supermassive black holes came to be as massive as they are today (and, in the most extreme cases, how they were able to rapidly do so while the universe was still in its relative infancy).
Two potential projects are available, on black hole spin and on ultraluminous X-ray sources, both of which will primarily involve analysis of both new and archival X-ray observations from e.g. the XMM-Newton, Swift, Chandra and NuSTAR observatories, including spectroscopic and time-domain analyses. In addition to the immediate scientific progress, both will also serve as key preparation for the Athena X-ray observatory – ESA’s next flagship-class space telescope – and also offer connections with the gravitational wave community.
Black Hole Spin: in astrophysics black holes are expected to be fully defined by just two numbers, their mass and their spin (or angular momentum). Information on the growth history of SMBHs is imprinted on their spin distribution, and so SMBH spin measurements are of particular importance for understanding this process, and in turn galaxy evolution (which is inextricably linked to SMBH growth). Such measurements are possible via detailed X-ray spectral/timing analyses of accreting black holes, but disparate analyses have been presented in the literature. The primary goal here is to use the latest, state-of-the-art spectral models to construct a pipeline to measure BH spin that can be applied uniformly to samples of AGN, a key step in connecting SMBH spin measurements to galaxy evolution models. Furthermore, the same techniques can be used to measure spin in black hole XRBs, offering a means to compare the XRB population with the black holes seen merging via gravitational waves.
Ultraluminous X-ray Sources: these are the most extreme XRBs and are now understood to be the best local examples of accretion above the Eddington limit, which may be required to grow the SMBHs now being seen in the early universe. ULXs were themselves assumed to be black holes for a long time, but remarkably we now know that some ULXs are powered by wildly super-Eddington neutron stars! Only a handful of these neutron star ULXs are currently known, but there is speculation they may make a significant contribution to the broader ULX population. In fact, we are still hunting for the first confirmed black hole ULX, although there is again speculation that such systems exist as evolutionary pre-cursors to the gravitational wave black hole mergers. The primary goal here is to further our understanding of the ULX population, studying the known ULX pulsars to understand how they are able to accrete at such extreme rates, searching for new ULX pulsars, and also for the first black hole ULX.