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.

23.05.2019

M. Vuckovic and A. Franeck

Joint journal club

Maja Vuckovic will present paper: Peece et al. 2019 MNRAS, "Convection physics and tidal synchronization of the subdwarf binary NY Virginis" http://adsabs.harvard.edu/abs/2019MNRAS.485.2889P and Annika France will present paper: Bonaca et al., 2019, ApJ, submitted: "The Spur and the Gap in GD-1: Dynamical evidence for a dark substructure in the Milky Way halo" https://arxiv.org/abs/1811.03631

27.05.2019

Prof. Svatopluk Civiš

Negative ions and their identification in laboratory and in interstellar space

Over 150 molecules and ions have been so far identified in the interstellar space. Ions play dominant role in the interstellar chemical reactions. In total 16 cations (CH+, CO+, SO+, HCO+, HCS+, HOC+, N2H+, H3+, H2D+, CH2D+, HCNH+, HOCO+, H3O+, H2COH+, HC3NH+) were found. Chemical reactions in the interstellar space are connected to reactions of cations, and the presence of anions was theoretically predicted. In 2006 their existence was confirmed observationally.

21.06.2019

Peter Boorman

Constructing a Census of Growing Supermassive Black Holes - First Results from the NuLANDS Survey

Sensitive X-ray observations have revealed that most growing supermassive black holes (aka Active Galactic Nuclei - AGN) are obscured by large columns of gas, with their broadband spectra peaking in the X-ray and IR regimes. As such, these regimes carry complementary information about the AGN intrinsic and reprocessing structures. In this talk, I will discuss multiple techniques of selection and characterisation of obscured AGN, including new results based upon an X-ray 'Baldwin Effect' in heavily obscured sources. This study indicates that simplistic modelling of reprocessed X-ray spectra in highly obscured AGN underpredicts their growth rates, and thus a ‘representative' census of the entire AGN population is essential. To tackle this issue, we have begun the NuSTAR Local AGN N(H) Distribution Survey (NuLANDS) — a > 4 Ms legacy survey of AGN currently underway with NuSTAR, XMM-Newton and the Neil Gehrels Swift Observatory. The novel IR selection and optical classification of the sample ensures we are unbiased by X-ray obscuration up to the highest columns of material. I will present the preliminary results from the survey, most notably the indication that > 30% of all AGN in the local Universe are heavily obscured, and that hard X-ray selection alone preferentially selects less-obscured AGN. These preliminary NuLANDS results thus mark a major step towards completing the local census of accretion activity amongst AGN, as well as understanding the role obscuration plays in their evolution.

22.07.2019

Emily Moravec

Radio-Active CoWS: Probing the Role of Environment on Radio-AGN at z~1 with MaDCoWS

AGN properties and dense environments are connected in a myriad of ways. For galaxies, the hot gas found in dense environments can impact AGN fueling and confine AGN outflows. Meanwhile, feedback from AGN jets can have a significant mechanical and thermal impact upon the surrounding gas. To better understand the interplay between AGN and the hot intracluster medium in galaxy clusters, we have been investigating the distribution of AGN and properties of the AGN population associated with z~1 galaxy clusters from the Massive and Distant Clusters of WISE Survey (MaDCoWS). I will first give an overview of MaDCoWS, which is the first cluster survey capable of discovering massive clusters at these redshifts over the full extragalactic sky, and will discuss key results from this program. I will then focus upon our studies of the AGN population. I will first discuss the distribution of AGN relative to the clusters. Then, I will present the results from a study with the JVLA to determine the radio morphologies of extended radio sources and the properties of their host-galaxies in a sub-sample of ~50 MaDCoWS clusters. Using optical, infrared, and radio data, we investigate correlations between the size and luminosity of the radio-AGN with stellar mass, cluster-centric radius, and cluster richness. We find a remarkable diversity of radio galaxy and host galaxy properties across a range of cluster properties.

26.07.2019

Tiina Liments and Boris Deshev

Joint journal club

Tiina Limets will present paper: Bujarrabal et al. 2018, A&A, 616, L3 "High-resolution observations of the symbiotic system R Aqr. Direct imaging of the gravitational effects of the secondary on the stellar wind" https://ui.adsabs.harvard.edu/abs/2018A%26A...616L...3B/abstract and Boris Deshev will present paper: by Zhang et al. (2019), submitted "On The Effect of Environment on Line Emission from the Circumgalactic Medium" https://ui.adsabs.harvard.edu/abs/2019arXiv190601643Z/abstract

29.07.2019

Lucie Tvrzníková

Rare event searches: looking for dark matter and new neutrino physics

In this talk, I will present a high-level overview of my work: In the first half, I will focus on direct dark matter (DM) searches and R&D efforts for the next generation of xenon time projection chambers. The nature of DM remains a mystery since it has so far eluded detection in the laboratory. The Large Underground Xenon (LUX) experiment was built to observe the interaction of DM with xenon target nuclei. LUX acquired data from April 2013 to May 2016 at the Sanford Underground Research Facility in Lead, South Dakota, which led to publications of many world-leading exclusion limits that probe much of the unexplored DM parameter space. I will discuss the results from the LUX detector with a focus on sub-GeV dark matter, the status of the next-generation detector known as LZ, and R&D efforts on high voltage characterization in liquid argon and xenon to enable future detector design. In the second part of my talk, I will focus on neutrino experiments trying to answer questions that will open the door to physics beyond the Standard Model: whether neutrinos are their own antiparticles and what is the effective mass of the electron antineutrino m_ve. This effective mass can be determined from a sufficiently high-precision measurement of the tritium beta-decay spectrum around its endpoint. I will introduce the Project 8 experiment, which is using the Cyclotron Radiation Emission Spectroscopy (CRES) technique to perform a radio-frequency-based measurement of the decay electron energy. To achieve its goal sensitivity of m_ve 40 meV, Project 8 will use an atomic tritium source to eliminate rotational and vibrational excitations of molecular tritium that perturb the tritium spectrum endpoint. I will discuss the recent efforts toward atomic tritium at Lawrence Livermore National Lab. Lastly, I will also discuss R&D efforts to characterize SeF6 to evaluate its effectiveness as a neutrinoless double-beta decay detector target.

12.09.2019

Koshy George

Ultraviolet view of star formation quenching in galaxies

Spiral galaxies like our own Milky Way maintains a constant star formation in the disk through continuous replenishment of gas supply from infall and accretion. The ongoing star formation in a spiral galaxy can however be quenched due to internal and external processes. AGN feedback (internal) and ram pressure stripping (external) are the two efficient processes that are hypothesised to be responsible for star formation quenching in spiral galaxies. This talk presents an ongoing ultraviolet survey of spiral galaxies in dense environments using ASTROSAT/UVIT where we see evidences of AGN feedback and ram pressure stripping simultaneously quenching star formation in a spiral galaxy.

20.09.2019

Matúš Rybák

Beyond blobs: dissecting extreme, dusty starbursts on (k)pc-scales

Sub-millimeter galaxies (SMGs) play a key role in the early the Universe and are an important laboratory for understanding intense star formation. However, our understanding of SMGs -- what drives their intense star-formation? what are the physical properties of their star-forming ISM? - requires high angular resolution, long unavailable in mm/FIR wavelengths. The start of ALMA operations in 2011 was a quantum leap from barely resolved SMGs to kpc-scales studies, matching or even exceeding the resolution achieved for present-day galaxies. A further order-of-magnitude jump in resolution - to parcsec scales - and sensitivity has been provided by strong gravitational lensing. I will showcase recent results from resolved multi-tracer (FIR, CO, C+) studies of dust and gas in z>2 SMGs at (k)pc resolution. Combining the superb angular resolution and high-frequency capabilities of ALMA, gravitational lensing and radiative transfer modelling, our results provide an unprecedented view of the conditions in these extreme star factories down to 50-pc scales

26.09.2019

Yanina Cochetti & Michalis Kourniotis

Joint journal club

Michalis Kourniotis will speak about the detection of the water in the atmosphere of the habitable-zone eight-Earth-mass exoplanet K2-18b. Based on papers: https://www.nature.com/articles/s41550-019-0878-9.pdf https://arxiv.org/pdf/1909.04642.pdf and Yanina Cochetti will present paper: Jorick S. Vink, A&A 619, A54 (2018) https://www.aanda.org/articles/aa/pdf/2018/11/aa33352-18.pdf

27.09.2019

Martin Macháček

A system of linear algebraic equations for the long-time mean velocities of fluid in stationary turbulence

First we transform the Navier-Stokes equations into a strictly equivalent but simpler dynamical system and show some of its applications. Then we examine the case of stationary turbulence and in a mathematically rigorous way derive a system of linear algebraic equations for long time mean values of fluid velocity at diffferent points and of their product, e. g. <v(x)>, <v(x) v(x')>, <v(x) v(x') v(x'')> etc.

30.09.2019

Cosimo Bambi

Testing general relativity using X-ray reflection spectroscopy

Einstein's theory of general relativity was proposed over 100 years ago and has successfully passed a large number of observational tests in weak gravitational fields. However, the strong field regime is still largely unexplored, and there are many modified and alternative theories that have the same predictions as Einstein's gravity for weak fields and present deviations only when gravity becomes strong. X-ray reflection spectroscopy is potentially a powerful tool for testing the strong gravity region around astrophysical black holes with electromagnetic radiation. In this talk, I will present the reflection model RELXILL_NK designed for testing the metric around black holes and the current constraints on possible new physics from the analysis of a few sources.

07.10.2019

Quentin Moreno

Particle-In-Cell, a numerical method at the border between astrophysics and laboratory experiments

The modern age of physics is to a large degree determined by the availability of high speed and high capacity computer systems. The use of these computing facilities for performing numerical experiments on collisionless plasmas covers now almost half a century of experience. Many problems in plasmas and in particular space plasma physics with their enormous complexity could not have been solved or even tackled without computers and numerical simulations. The latter must accompany observations and experiments in order to understand what is going on in such non-linear plasma physics. A brief discussion about numerical methods is therefore not only unavoidable but even necessary. Thus far, two kinetic numerical approaches are commonly used in the plasma physics community to study collisionless plasma processes. The Vlasov approach based on an Eulerian scheme is one of them. The code performs a direct integration of the Vlasov-Maxwell system of equations discretized on a phase-space grid. Another approach is the Particle-In-Cell method (PIC) using a Lagrangian-Eulerian approach, where the distribution function of each species is formed by a collection of computational particles (CPs), and Maxwell’s equations are solved on an Eulerian grid. Such codes can be used to excite linear plasma modes and nonlinear plasma dynamics can be followed. Our presentation is dedicated to PIC simulation code and is structured as follows. We outline the principle upon which a PIC code is based in a first part, and we summarize the numerical Yee leap-frog scheme. A second part is dedicated to present the Open Source simulation code SMILEI, where examples will be illustrate in a third part.

11.10.2019

Petr Kubánek

Software Engineering at LBTO

Large Binocular Telescope Observatory (LBTO) is one of the largest currently operating telescope on the planet (and the largest one in continental USA). Boasting two 8.4m Richard F. Caris Mirror Lab honeycomb mirrors, perched atop Mount Graham in Southeast Arizona, it comes equipped with the state of the art sensors and actuators. Thanks to its partly experimental design, the telescope has adaptive optics secondary mirrors, multiple focal stations and a growing list of instruments. In this talk I will focus on design of the software to run the observatory. Staring from the low level hardware interfaces, I will progress towards the upper layer used by observers to control the observations. I will as well describe some of the science done with its instruments, as well as some of the unique observations achieved by this complex instrument.