Venturing Beyond the ISCO: Probing the black hole plunging region

17th American Astronomical Society HEAD Meeting, Monterey, CA (20/03/2019)

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. General relativity predicts that in close proximity to the black hole, within the innermost stable circular orbit (ISCO), gravity is sufficiently strong that stable circular orbits cannot exist and upon reaching this radius, material within the accretion disc must plunge into the black hole. In recent years, the advent of X-ray timing studies has revealed unprecedented amounts of information about the extreme environments around black holes. In particular the detection of the coronal X-ray emission reverberating off the inner regions of the accretion disc has revealed the geometry and the dynamic nature of the corona. The next step is to detect the material within the ISCO and understand its dynamics as it plunges into the black hole. Analysis typically assumes that no X-rays are detected from material within the ISCO. General relativistic ray tracing simulations, however, 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 of X-ray observatories such as Athena, and specialized X-ray timing missions such as STROBE-X. 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 and would be an important component in validating black hole spin measurements. 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.