15.01.2019

Petr Závada

Statistical analysis of binary stars from the Gaia catalogue DR2

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.

25.01.2019

Mark Morris

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

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.

01.02.2019

Agnieszka Janiuk

Black Hole accretion and jet ejection in Gamma ray bursts

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.

11.02.2019

Roberto Oliveri

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

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.

21.02.2019

Maica Clavel

Recent breakthroughs in our understanding of Sagittarius A*’s past activity

The supermassive black hole at the center of the Milky Way, Sagittarius A*, is currently one of the least luminous known supermassive black holes. However, X-ray observations carried out over the past decade have provided strong evidence that Sgr A* has experienced periods of higher activity in the past centuries. By tracing the echoes of these past outbursts as they propagate through the Galactic center molecular clouds, we intend to better understand the duty cycle of this supermassive black hole and to identify the corresponding catastrophic events. Yet, reconstructing Sgr A*’s past light curve remains challenging: I will review results and perspectives of this relatively new astro-archeology research field.

08.03.2019

Areti Eleni

Studying orbits around a Kerr black hole using a Newtonian analogue

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.

15.03.2019

Ladislav Šubr

Intermediate-Mass Black Holes in binary-rich star clusters

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.

22.03.2019

Alexis Finoguenov

The infall of X-ray groups on to massive clusters

Galaxy clusters are expected to form hierarchically in a Λ cold dark matter (ΛCDM) universe, growing primarily through mergers with lower mass clusters and the continual accretion of group-mass haloes. Galaxy clusters assemble late, doubling their masses since z ˜ 0.5, and so the outer regions of clusters should be replete with accreting group-mass systems. We present an XMM-Newton survey to search for X-ray groups in the infall regions of 23 massive galaxy clusters (<M_200 > ˜ 10^15M_⊙ ) at z ˜ 0.2, identifying 39 X-ray groups that have been spectroscopically confirmed to lie at the cluster redshift. These groups have mass estimates in the range 2 × 10^13 -7 × 10^14 M_⊙ , and group-to-cluster mass ratios as low as 0.02. The comoving number density of X-ray groups in the infall regions is ˜25× higher than that seen for isolated X-ray groups from the XXL survey. The average mass per cluster contained within these X-ray groups is 2.2 × 10^14 M_⊙ , or 19 ± 5 per cent of the mass within the primary cluster itself. We estimate that ˜10^15M_⊙ clusters increase their masses by 16 ± 4 per cent between z = 0.223 and the present day due to the accretion of groups with M_200 ≥ 10^13.2M_⊙ . This represents about half of the expected mass growth rate of clusters at these late epochs. The other half is likely to come from smooth accretion of matter not bound within haloes. The mass function of the infalling X-ray groups appears significantly top heavy with respect to that of `field' X-ray systems, consistent with expectations from numerical simulations, and the basic consequences of collapsed massive dark matter haloes being biased tracers of the underlying large-scale density distribution.

04.04.2019

Norbert Werner

X-ray spectroscopy of the atmospheres of galaxies, clusters of galaxies, and of planets in our Solar system

Most galaxies comparable to or larger than the mass of the Milky Way host hot, X-ray emitting atmospheres, and many such galaxies are radio sources. Hot atmospheres and radio jets and lobes are the ingredients of radio-mechanical active galactic nucleus (AGN) feedback. While a consensus has emerged that such feedback suppresses cooling of hot cluster atmospheres, less attention has been paid to massive galaxies where similar mechanisms are at play. I will review the results of X-ray observations of galactic atmospheres, including their chemical composition, dynamics, development of thermal instabilities, and AGN heating. Finally, I will switch the attention to the atmospheres of the planets in our Solar system and introduce the use of high resolution X-ray spectroscopy in the studies of Mars, Jupiter and Saturn.

05.04.2019

Ivan Hubený

Radiative transfer and models of vertical structure of accretion disks

I will briefly summarize various approaches to treat radiation and radiative transfer in accretion disks, and will show several examples how the radiation influences the vertical structure of a disk. Although the emphasis is given on black hole disks, I will also briefly discuss disks around neutron stars (X-ray binaries) and white dwarfs (cataclysmic variables).

24.04.2019

Sac Nicté Medina

Characterization of Radio sources in the ISM

In observational astronomy, studying the interaction between the molecular and the ionized gas requires a first step that is to characterize the emission, which is not a simple work. In this talk, I will mention important actions to characterize the radio and millimeter emission from star-forming regions. The used data include VLA C-band (4-8 GHz) and IRAM 30 m 13CO(2-1) emission (220.399 GHz) observations, while the actions are from the source extraction to the use of statistical methods. I will describe the source extraction and nature of the radio sources from the well known star-forming region NGC 6334, and from the GLOSTAR-VLA survey. I will also explain the characterization of the turbulence profile of the molecular gas from 13CO emission of a sample of the SEDIGISM survey and the W43 HERO program. The results from these works are fundamental to study the interaction between the molecular and the ionized gas with observational data.