Self-similar accretion in thin discs around near-extremal black holes

Roberto Oliveri

11.02.2019 14:00

Near-maximally spinning black holes display conformal symmetry in their near-horizon region, which is therefore the locus of critical phenomena. In this talk, we revisit the Novikov–Thorne accretion thin disc model and find a new self-similar radiation-dominated solution in the extremely high spin regime. Motivated by the self-consistency of the model, we require that matter flows at the sound speed at the innermost stable circular orbit (ISCO). We observe that, when the disc pressure is dominated by radiation at the ISCO, which occurs for the best-fitting Novikov–Thorne model of GRS 1915+105, the Shakura–Sunyaev viscosity parameter can be expressed in terms of the spin, mass accretion rate and radiative efficiency. We quantitatively describe how the exact thin disc solution approaches the self-similar solution in the vicinity of the ISCO and for increasing spins.

Intermediate-Mass Black Holes in binary-rich star clusters

Ladislav Šubr

08.03.2019 11:00

There is both theoretical expectation and some observational clues that intermediate mass black holes reside in nuclei of globular clusters. In order to find an independent indicator for their existence, we investigate in this paper how an IMBH manifests itself through its dynamical interaction with a binary rich globular cluster of moderate extension and mass. By means of direct N-body integration we follow the dynamical evolution of models of such a system over a time span of ≈ 0.8 { Gyr} and compare the cases with and without the primordial binaries as well as with and without the IMBH. In accord with previous results, we show that when present the IMBH develops a power-law density cusp of stars around it, regardless of the binary population in the cluster. If, however, binaries are present, their interaction with the IMBH leads to the production of high velocity escapers at a rate of the order of 0.1 { Myr}^{-1}. These stars may contribute to the population of high-velocity stars observed in the Galaxy. Clusters hosting the IMBH together with high number of binaries also form a denser halo of marginally unbound stars than clusters that lack either the IMBH or the rich binary population. Finally, we show that the binary population leads to an increased rate of direct interactions of stars with the IMBH, potentially observable as tidal disruption events.

Studying orbits around a Kerr black hole using a Newtonian analogue

Areti Eleni

15.03.2019 11:00

In 1760 Euler studied the problem of the motion of a test particle in the gravitational field of two fixed mass centers. When the distance between the two centers is purely imaginary, the corresponding gravitational potential could remain real and the geometrical structure of its field becomes oblate. This field turns out to share a lot of similarities with the Kerr black holes. Although the framework behind the two objects is completely different, both problems refer to gravitational fields that have quite intriguing analogies with respect to orbital motions of a test-body in them. In my talk I will demonstrate the similarities between the two problems and I will discuss the possibility to use the Newtonian problem to get insight into cases where the relativistic treatment of the field of a Kerr black hole becomes very complicated.

TBD

Alexis Finoguenov

22.03.2019 11:00