Simulating relativistic effects in tidal disruptions of stars by rotating supermassive black holes

by Mr Emanuel Gafton (Stockholm University; The Oskar Klein Centre)

Tuesday 4 Apr 2017, 15:30 → 16:30 Europe/Stockholm

FC61

The supermassive black holes that reside in the centres of most known galaxies are able to disrupt solitary stars that approach them on sufficiently close orbits. The observational signatures of such encounters may be used to probe the properties of the black hole, to test strong gravity and accretion theories, and to explain a number of puzzling observations around Sgr A*. Although a general picture of a typical tidal disruption can be easily obtained from analytical estimates, such an event is ultimately governed by the complex interplay between hydrodynamics, self-gravity, and the strong gravity (with relativistic effects) of the Kerr black hole, and many questions can only be answered with the aid of numerical simulations. In this talk I will focus on a new method that we have developed for accurately simulating the relativistic effects of the central black hole. After a brief overview of the standard, Newtonian picture of a tidal disruption, I will discuss in which regime relativistic effects should be included in a simulation, how they have been modelled in previous works (e.g., by means of pseudo-relativistic potentials, or by combining various methods for the various stages of the disruption), and then I will describe our new method, which –at a virtually insignificant computational cost as compared to a Newtonian simulation– includes exact relativistic tidal forces and hydrodynamics, and a very good approximation for the self-gravity of the star. After the presentation of the method and of various tests, I will show the results of simulations designed to study the effect of the black hole spin on the morphology and fallback rate of the debris streams, finding that while the spin has little effect on the latter, it does imprint heavily on the stream morphology, and can even be a determining factor in the survival or disruption of the star itself.