Venturing beyond the ISCO: Mapping the extreme environments around black holes

30th Texas Symposium on Relativistic Astrophysics (16/12/2019)
JSI Workshop ‘The New Faces of Black Holes,’ Annapolis, MD (11/11/2019)

The immediate vicinities of black holes represent some of the most extreme environments in the Universe, where accreting material in its final moments before plunging through the event horizon powers some of the most luminous sources in the Universe; bright X-ray emitting coronae and vast jets launched close to the speed of light. 

The advent of X-ray timing studies has revealed unprecedented amounts of information about the extreme environments around black holes. The reverberation of the coronal X-ray emission off the inner regions of the accretion disc allows the region outside the event horizon to be mapped. X-ray reverberation reveals the geometry and the dynamic nature of the corona. The structure of the corona is revealed, with the discovery of a collimated core, reminiscent of a jet embedded within an extended corona, along with how these components evolve to give rise to the extreme X-ray variability that is observed.

The next step is to detect and understand what happens to material after it crosses innermost stable circular orbit (ISCO), predicted by General Relativity where gravity is sufficiently strong that stable circular orbits cannot exist. General relativistic ray tracing simulations show how signatures of material inside the ISCO, plunging into the black hole, are manifested in observations of X-ray reverberation. Simulations reveal how emission specifically reverberating off of material in the plunging region may be detected with the next generation X-ray observatories.

The ability to directly detect the presence of an innermost stable orbit and plunging region would provide a unique test of general relativity in the strong field limit, only accessible around black holes. Probing the dynamics of material in the plunging region will reveal how the accretion flow behaves in its final moments and how it may launch jets, accelerate coronae and power some of the most extreme systems in the Universe.