Problems in the theory of black hole accretion

Lead Research Organisation: University of Oxford
Department Name: Oxford Physics


Despite its historical roots, the problem of exactly what happens when gas accretes onto a black hole is not well-understood in detail. Historically, observations of black hole accreting gas was the proxy by which the very existence of black holes was to be settled. In addition to the inherent complexity of the turbulent flow itself, the effects of environmental history in the form of initial conditions (especially with the magnetic field) make detailed predictions extremely difficult. When a "typical" black hole luminosity is needed for applications (e.g. as a constraint on black hole numbers), all too often the alternative used is a gross over-simplification, such as spherically symmetric Bondi accretion.

In this project we will revisit this classic problem, aided by several new tools. The first is the use of a 1D disc evolution equation, analogous to the Lynden-Bell-Pringle equation of classical disc theory, but valid in the relativistic regime for Kerr black holes. This extremely useful equation will be applied to the study of Tidal Disruption Events (destruction of a passing star by the tidal forces of a massive black hole), whose X-ray light curves show a power law fall-off in time which is much too shallow when compared with the predictions of simple matter "fall-back" theories. Preliminary analysis suggests that this could be understood by imposing a finite inner stress boundary condition in a debris disc surrounding the hole. We will work out these models in detail using pencil and paper mathematical analysis, numerical simulations of the 1D evolution equation, and fully 3D solutions of general relativistic magnetised disc models. The latter will be carried out using the ATHENA code, in collaboration with Prof J Stone of Princeton University , where Mummery will spend an extended period of time in academic 2019-20. (Prof Stone has agreed to this arrangement.) The ATHENA code is currently in the process of having a radiation module added to its capabilities, so that the timing is ideal for pursuing emission calculations and observational signatures in the later stages of this work.


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