Seminars archive

Statistical analysis of binary stars from the Gaia catalogue DR2

Petr Závada

We have developed a general statistical procedure for analysis of 2D and 3D finite patterns, which is applied to the data from recently released Gaia-ESA catalogue DR2. The 2D analysis clearly confirms our former results on the presence of binaries in the older DR1 catalogue. Our main objective is the statistical 3D analysis of DR2. For this, it is essential that the DR2 catalogue includes parallaxes and data on the proper motion. The analysis proves a high rate of binaries in the region under study and allows us to estimate a high limit of their separation: ~0.1pc. Finally and most importantly, we had shown that combined analysis of the separations with proper motion of the pairs of sources provides a clear picture of binaries with two components of the motion: parallel and orbital. The analysis allowed us to estimate the average orbital period and mass of the binary star system in a chosen statistical ensemble.

New Results on the Galactic Center at Very Small and Very Large Scales

Mark Morris

Using adaptive optics observations with the Keck Telescopes, the UCLA Galactic center group has followed the orbit of the star S0-2 through its periapse passage close to the Galactic Black Hole. I will show how the radial velocity and astrometric measurements of this star’s orbit show good agreement with both special and general relativistic predictions (Tuan Do et al. 2019). Then, turning to very large scales, I’ll describe X-ray mapping observations of the Galactic center with XMM-Newton that show a “chimney” of X-ray emission centered on Sgr A* and extending a few hundred parsecs perpendicular to the Galactic plane in both directions. We (Gabriele Ponti et al. 2019) interpret this as a channel containing a hot plasma generated by activity associated with the Galactic Black Hole. This channel extends to the gamma-ray-emitting Fermi Bubbles, and may be the conduit through which relativistic particles travel to energize those gigantic features.

Black Hole accretion and jet ejection in Gamma ray bursts

Agnieszka Janiuk

Short gamma ray bursts are presumably results of binary neutron star mergers, which lead to the formation of a stellar mass black hole, surrounded by a remnant matter. The strong magnetic fields help collimate jets of plasma, launched along the axis of the black hole rotation. We study the structure and evolution of the accreting plasma in the short GRBs and we model the formation of the base of a relativistic, Poynting-dominated jets. Our numerical models are based on the general relativistic MHD, axisymmetric simulations. In this talk I will discuss the origin of variability in the GRB jet emission, which timescales are related to the action of the magneto-rotational instability in the accreting plasma, and the maximum achievable Lorentz factor is determined by the magnetic energy flux along the field line. I will also briefly describe the process of nucleosynthesis in the uncollimated GRB outflows and the imprint of r-process elements on their observable properties.

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

Roberto Oliveri

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

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

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