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.

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.