30.01.2025

M. Kraus & R. Taylor

Joint Journal Club

Joint Journal Club in Sporilov. Two papers will be presented: - From GPS/ASU, Rhys Taylor will present the paper "Dark-matter-free Dwarf Galaxy Formation at the Tips of the Tentacles of Jellyfish Galaxies" - https://ui.adsabs.harvard.edu/abs/2024ApJ...969...24L/abstract[TBD] - From STE/ASU, M. Kraus will present the paper "The ESO UVES/FEROS Large Programs of TESS OB pulsators: II. The physical origin of macroturbulence" - https://ui.adsabs.harvard.edu/abs/2024A%26A...692A.245S/abstract Zoom link is distributed separately.

03.03.2025

Thibault Barnouin

The long awaited come back of UV polarimetry of Active Galactic Nuclei

The last UV detectors capable of measuring the polarized signals from Active Galactic Nuclei (AGN) were the Faint Object Spectrograph (FOS) and Faint Object Camera (FOC) on board the Hubble Space Telescope (HST), decommissioned in 1997 and 2001, respectively, and the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), which flew on two NASA Space Shuttle missions in December 1990 and March 1995. The FOC’s imaging resolution of 22x22 milliarcseconds squared at wavelengths as short as 115 nm is still unmatched to this day. Used together with polarizing filters, it allowed the study of 25 AGN, providing great insight into the structure of the scattering medium of type 2 AGN and the precise location of their obscured nucleus. The FOS's far- to near-UV high spectral resolution polarimeter — achieved using a prism and rotating waveplate — enabled the study of 50 AGN. During its only two flights, WUPPE obtained spectropolarimetry for only 5 AGN. With the exception of one radio-quiet AGN (NGC 1068), WUPPE's AGN observations had very poor spectral resolution in polarimetry. Although limited in number, these observations provided critical insights into the physics of these highly asymmetrical and spatially unresolved objects. During this presentation, I will go through the AGN samples in the polarimetric archives of HST/FOC, HST/FOS, and WUPPE, highlighting their groundbreaking contributions and the unanswered questions that remain since their decommissioning. Finally, I will discuss how a POLLUX-like instrument — to be proposed by CNES as a fourth instrument for NASA’s Habitable Worlds Observatory — could not only build upon this legacy but also surpass it.

02.04.2025

Ondřej Pejcha

Star Dance: From Wide Pairs to Collisions

Our Sun is a single star, which is somewhat of an exception in the Universe as most stars are found in binary and multiple systems. For a wide range of initial conditions at stellar birth, the two stars in a binary will interact by exchanging mass, exploding, or merging to a single object. Computer simulations of binary systems are indispensable in revealing the fundamental physical processes governing the binary's structure and evolution, yet they are complicated by a lack of any useful symmetry. In this talk I will review results of radiation / magneto-hydrodynamical simulations of a common envelope evolution phase studied with the help of ERC StG and describe plans for a new set of simulations with discontinuous Galerkin method to be done with the support of ERC CoG.

08.04.2025

Harrison Souchereau

Inner Tail Gas Asymmetries and Fallback in the Jellyfish Galaxy NGC 4858

Gaseous evolution due to ram pressure acceleration of material that has been pushed out of the disk, but not fully stripped, can be a complex process. We explore this ”inner tail” evolution using high-resolution (∼1” = 460pc) ALMA CO(2-1) observations of the Coma cluster jellyfish galaxy NGC 4858, obtained from the ALMA-JELLY large program. We compare this spectacular observational data to a suite of state of the art ENZO ”wind tunnel” galaxy simulations, where we vary the disk-wind angle between runs. In our observations, we find numerous structural and kinematic features indicative of the effects from strong, inclined ram pressure, including an asymmetric inner gas tail. We also find kinematic signatures of fallback: gas clumps that had been previously pushed out of the disk but are now falling inwards. We propose a simple, torque-focused mechanism that would create asymmetric inner-tail morphologies regardless of the presence of pre-existing spiral structure. These asymmetries are supported by the results of our simulation suite which also confirm where fallback is to be expected based on the galaxy’s rotation and the ram pressure wind direction. We find that while fallback occurs in all simulations more inclined than 45 degrees, the fallback rates are largest for angles closer to edge-on. We also find that star formation is elevated for more inclined winds, and can lead to the generation of a ”leading edge plume”: shell-like structures of stars upstream from the galaxy disk.

24.04.2025

L. Ferrero and P. Jachym

Joint Journal Club

Joint Journal Club Two papers will be presented: - From STE/ASU, Leticia Ferrero will present the paper: "A kinematical study of the launching region of the blueshifted HH 46/47 outflow with SINFONI K-band observations", Birney et al. 2024 https://ui.adsabs.harvard.edu/abs/2024A%26A...692A.143B/abstract - From GPS/ASU, Pavel Jachym will present [TBD] Zoom link is distributed separately.

07.05.2025

Daniela Pérez

Survival of wormholes in a cosmological bounce

The standard cosmological model, called the Λ-CDM model, is a 6-parameter model that includes Einstein’s gravity with the cosmological constant, baryons, neutrinos, photons, cold dark matter, and a hot initial state. Though it works very well to explain the available astronomical data, it is, nonetheless, not free of problems. One major issue is that the ΛCDM model is singular, a feature that is inherent in the incompleteness of the background theory, in this case general relativity. Another approach to avoid the singularity is through a cosmological bounce from a previous contracting phase. During the contraction, the density and temperature of the cosmic fluid increases in such a way that all structure is expected to be erased. However, it has been shown that black holes can survive the bounce. Another type of object that might survive a bounce is a wormhole. Although these are conjectural objects, the potential astrophysical effects caused by them have been studied extensively. In this talk, I will show that there is one solution of Einstein's field equations that represents a dynamical wormhole that survives a cosmological bounce. I will also discuss some possible astrophysical manifestations of surviving wormholes in a bouncing universe.

20.05.2025

Mark Morris

Origin and Manifestations of The Galactic Wind

Galactic winds can be seen in a variety of nearby spiral galaxies, most clearly if those galaxies are edge-on to our line of sight. Such winds usually emanate primarily from the galaxies’ central regions. But the recognition of a central wind blowing outward from our Milky Way Galaxy into the Galactic halo has become evident only in the last few decades, perhaps because we are “too close” to the action to easily see the full picture. In this talk, I will describe the various known manifestations of the Galactic wind, starting with the earliest observations of the hot, coronal wind using absorption lines against distant quasars. Other manifestations of the wind include radio continuum and radio line emission, gamma-ray emission from the gigantic Fermi Bubbles, and an X-ray emitting plasma, extending from a central “chimney” on scales of hundreds of parsecs all the way out to the eRosita X-ray bubbles on a scale exceeding 10 kpc. But what is the origin of the Galactic wind? I will present evidence that there are at least two sources, the dominant ones being the collectivity of supernovae occurring in the Galaxy’s Central Molecular Zone, and separately in the 3 to 5 kpc ring of star formation in the inner regions of the Galactic disk. Of course, episodic outflows from the Galactic black hole are probably also a non-negligible contributor during periods of enhanced accretion. Finally, I will suggest what the next steps might be for the exploration of the Galactic wind.

23.05.2025

Michal Bílek

Fornax dwarf spheroidal an its globular clusters in MOND

The presence of globular clusters (GCs) in Fornax dwarf is important for testing different dark matter and modified gravity theories. The problem is that in most of them, dynamical friction acting on the GCs is so strong, that the GCs would settle in the center of Fornax faster than is the observed stellar age of GCs. I will present our high-resolution N-body simulations aiming to answer whether the GCs of Fornax are consistent with the MOND modified gravity. Following older MOND studies of the Local Group, we assumed that Fornax is a tidal dwarf galaxy formed in a Milky Way - Andromeda flyby 7.5 Gyr ago. We newly propose that the GCs of Fornax started their lives as massive star clusters in the young Milky Way. Our results are the following: 1) The scenario can explain why the Milky Way has a disk of satellites, whose member Fornax is. 2) The simulations show that the probability that Fornax has five surviving GCs, as observed, is 20%. 3) They also explain the observe spatial distribution of the GCs around Fornax. 4) For the latest proper motion, MOND gravity implies an orbit of Fornax whose perigalacticons perfectly agree with the observed peaks of star formation in Fornax and with the independently determined time of the Milky Way - Andromeda flyby. 5) Without any tuning, the simulations reproduce the observed photometric "halo" component of Fornax. 6) Fornax is predicted to have tidal tails which are at the current observational limits. 7) By an additional simulation, we show that if Fornax started its life as a rotating disky tidal dwarf galaxy, the varying tidal forces from the Milky Way were able to transform it into a non-rotating spheroid.

04.06.2025

Chand Swadesh

Accretion Geometry of Black Hole X-ray Binaries in the Hard State

Black hole X-ray binaries (BHXRBs) serve as ideal laboratories for studying the complex interplay between accretion and ejection processes in strong gravitational fields. Most of the low-mass BHXRBs are transient in nature and exhibit sporadic outbursts. During a typical outburst, these systems transition through various spectral states, from hard to soft, each associated with distinct spectral and timing properties. Despite decades of studies, the exact physical picture of these systems in the hard spectral state remains an open question, particularly regarding the geometry of the corona and extent of the inner accretion disk. While some studies argue for a disk that extends close to the innermost stable circular orbit (ISCO) in the hard state, others propose a significantly truncated disk scenario. In this talk, I will present findings from some of our recent broadband spectral and timing studies of BHXRBs in their hard state, which provide insights into the nature and evolution of the inner accretion flow.

09.06.2025

Benedikt Bergmann

Timepix detectors in Space: From radiation monitoring in low earth orbit to astroparticle physics

Hybrid pixel detectors (HPD) of Timepix [1,2] technology have become increasingly interesting for space applications. While up to date, common space radiation monitors rely on silicon diodes, achieving particle (mainly electron and proton) separation by pulse-height analysis, detector stacking, shielding or electron removal by a magnetic field, the key advantage of HPDs is that, in addition to the energy deposition measurement, particle signatures in the sensor are seen as tracks with a rich set of features. These track characteristics can be exploited for identification of particle type, energy, and its trajectory. Determining these pieces of information on a single layer bypasses the need for sensor stacking or complex shielding geometries, so that HPD based space radiation devices provide science-class data with a large field of view at an order of magnitude lower weight and approximately half of the power consumption compared with commonly used space radiation monitors. The first Timepix (256 x 256 pixels, 55 µm pitch) used in open space is SATRAM (Space Application of Timepix Radiation Monitor), attached to the Proba-V satellite launched to low earth orbit (LEO, 820 km, sun-synchronous) in 2013. During this time, it has been providing data for mapping out the fluxes of electrons and protons trapped in the Van-Allen radiation belts [3], e.g. by in-orbit maps of the ionizing dose rate dominated by electrons in polar horn regions, as well as protons in the South Atlantic Anomaly. Noiseless detection of individual particles allows to measure even rare signatures of highly ionizing events from galactic cosmic rays. I will discuss different data analysis methodology, relying on extraction and characterization of track features, novel machine learning approaches (e.g., [5]) and using statistical interpolation. Based on the success of SATRAM, advanced and miniaturized space radiation monitors based on Timepix3 [2] and Timepix2 [4] technology have been developed for the European Space Agency (ESA). These will be flown on the GOMX-5 mission (launch in 2023) and used within the European Space Radiation Array. Large area Timepix3 detectors (512 x 512 pixels, 55 µm pitch) were developed for the demonstrator of the penetrating particle analyzer [5] (mini.PAN), a compact magnetic spectrometer (MS) designed to measure the properties of cosmic rays in the 100 MeV/n – 20 GeV/n energy range in deep space with unprecedented accuracy, thus providing novel results to investigate the mechanisms of origin, acceleration and propagation of galactic cosmic rays and of solar energetic particles, and producing unique information for solar system exploration missions. Precise per-pixel time measurement together with a high spatial segmentation allows its use a single layer Compton camera, thus making it an interesting tool also for directional gamma-ray detection and polarization measurement.

09.06.2025

Farida Sial

social seminar by Afghan journalist

The speaker has worked as a journalist with Radio Azadi in Prague (part of Radio Free Europe/ Free Liberty) for nearly three years. Before this role, she spent several years in Afghanistan, where she worked with various media outlets. Her journalistic focus has consistently centered on critical issues such as women’s rights, the challenges and restrictions faced by women, unemployment, migration, human rights, and the ongoing humanitarian crises in Afghanistan. At Radio Azadi, her reporting has shed light on underreported topics within Afghan media, including human rights violations, arbitrary arrests, restrictions on girls’ education, limitations faced by Afghan youth, and other urgent social matters. The speaker holds a Bachelor’s degree in journalism and a Master’s degree in international relations. As a representative of Radio Azadi, she actively participates in international conferences that focus on issues affecting Afghan women. She remains deeply committed to amplifying the voices of Afghan women and all individuals who are deprived of their fundamental human rights.

13.08.2025

Diego Calderon

The formation and stability of a cold disc made out of stellar winds in the Galactic centre

The reported discovery of a cold (~10,000 K) disc-like structure within the central 5 × 10‑3 pc around the super-massive black hole at the centre of the Milk Way, Sagittarius A* (Sgr A*), has challenged our understanding of the gas dynamics and thermodynamic state of the plasma in its immediate vicinity. State-of-the-art simulations do not agree on whether or not such a disc can indeed be a product of the multiple stellar wind interactions of the mass-losing stars in the region. Here we aim constrain the conditions for the formation of a cold disc as a natural outcome of the system of the mass-losing stars orbiting around Sgr A*, to investigate whether the disc is a transient or long-lasting structure, and to assess the validity of the model through direct comparisons with observations. We performed a set of hydrodynamic simulations of the observed Wolf-Rayet (WR) stars feeding Sgr A* using the finite-volume adaptive mesh refinement code Ramses. We focus, for the first time, on the impact of the chemical composition of the plasma emanating from the WR stars. The simulations show that the chemical composition of the plasma affects the radiative cooling to a sufficient degree to impact the properties of the medium, such as density and temperature, and, as a consequence, the rate at which the material inflows onto Sgr A*. We demonstrate that the formation of a cold disc from the stellar winds is possible for certain chemical compositions that are consistent with the current observational constraints. However, even in such cases, it is not possible to reproduce the reported properties of the observed disc-like structure, namely its inclination and the fluxes of its hydrogen recombination lines. We conclude that the stellar winds alone are not sufficient to form the cold disc around Sgr A* inferred from observations. Either relevant ingredients are still missing in the model, or the interpretation of the observed data needs to be revised.

04.09.2025

S. Ehlerova & J. Fisak

Joint Journal Club

Joint Journal Club at GPS/ASU. Two papers will be presented: - From GPS/ASU, Sona Ehlerova will present the paper by Guan-Hong et al. 2025, "Stellar Population and Energy Feedback in the Supergiant Shell LMC 1" - https://ui.adsabs.harvard.edu/abs/2025AJ....169..283L/abstract - From STE/ASU, Jakub Fisak will present the paper by Debnath et al. 2024, "2D unified atmosphere and wind simulations of O-type stars" - www.aanda.org/articles/aa/abs/2024/04/aa48206-23/aa48206-23.html Zoom link is distributed separately.

10.09.2025

Angelica Albertini

Gravitational waves from large-mass-ratio black hole binaries (PhD Thesis Discussion)

This thesis focuses on the development of gravitational wave (GW) templates for inspiralling black hole binaries in the large-mass-ratio regime, which will be key sources for the next generation of GW detectors. The work is based on the effective-one-body (EOB) formalism, a powerful framework that maps the two-body problem in general relativity into the motion of a single body onto an effective metric. Such mapping enables the incorporation of post-Newtonian results, along with inputs from numerical relativity and gravitational self-force (GSF) theory. Building upon the EOB model TEOBResumS, originally developed for comparable-mass binaries, this thesis extends its applicability to large-mass ratio systems. Such extension has been achieved through systematic comparisons against GSF results, and the resulting model has been implemented into the TEOBResumS public infrastructure. This thesis, therefore, contributes to the modeling of compact binaries in a regime that is still largely unexplored, advancing our ability to decode the harmonics that we will be able to observe in the upcoming future.

12.09.2025

Akshay Singh

Exploring the Dynamics of Magnetically Arrested Disks: The Role of Radiative Cooling

Accretion disks are essential for understanding the dynamics of gas around black holes. The magnetically arrested disk (MAD) state, where the magnetic flux near the event horizon becomes saturated, has garnered significant attention following observations of supermassive black holes in M87 and Sagittarius A* by the Event Horizon Telescope (EHT) collaboration, which suggest that this is the preferred accretion state for such systems. In particular, low-luminosity systems like Sagittarius A* are significantly influenced by radiative cooling processes, which profoundly affect the thermal, magnetic, and dynamical properties of the accretion disk. In this talk, I will describe how radiative cooling impacts the structure and behaviour of MADs at sub-Eddington accretion rates. We analytically identify a critical mass accretion rate below which synchrotron radiation becomes a dominant cooling mechanism, altering the disk's thermal equilibrium and the MAD parameter. Using general relativistic magnetohydrodynamic (GRMHD) simulations from our massively parallel code cuHARM, I will explore how these cooling effects influence force balance, magnetic saturation, and jet efficiency for a range of black hole spins and accretion rates.

17.09.2025

Joseph Saji

Jet Dynamics and Heavy Element Nucleosynthesis in Compact Object Mergers

Short gamma-ray bursts (sGRBs) provide a window into the physics of relativistic jets launched from compact mergers. We investigate the structure and dynamics of these jets through GRMHD simulations, with a targeted study of GRB 090510. The models reproduce the observed energetics, variability timescales, and opening angle within uncertainties, while also probing the impact of surrounding dynamical ejecta on jet collimation. Complementary simulations of black hole–disk systems with the HARM-EOS code explore the nucleosynthesis of heavy elements in merger-driven outflows. Post-processing the disk winds with the SkyNet nuclear reaction network yields detailed r-process abundance patterns, including lanthanides and actinides relevant for kilonova emission. Taken together, these results connect the properties of short GRB jets with the nucleosynthetic role of compact object mergers in producing the universe’s heaviest elements.

23.09.2025

Omkar Bait

Probing the nature of Lyman continuum emitting and low-metallicity galaxies using radio observations

The nature of sources that led to the last phase transition of matter in the Universe, namely the cosmic reionization, remains an open question in astrophysics. It is widely believed that low-mass star-forming galaxies (emission lines galaxies, Lyman alpha emitters and related objects) are the major contributors to reionization. Thus, they have been intensely studied at multiple wavelengths using state-of-the-art facilities including HST, 10m-class telescopes and others. Recently the first large sample of Lyman-Continuum (LyC) emitting galaxies has been found at low redshift (z~0.3) with HST and the SDSS, revealing objects with strong ionizing photon production and high LyC photon escape fractions (fesc), which are cosmologically relevant, and with properties making them the best analogues of the sources of cosmic reionization. To better understand the nature, the physical processes at play, and the multi-wavelength SEDs of these interesting galaxies we have undertaken the first radio continuum (RC) observations of such objects. I will present the results from this study using the Very Large Array (VLA) at multiple radio bands (1-8 GHz). Their radio-spectral energy distribution (SED) at GHz shows a wide variety along with evidence for spectral steepening and free-free absorption. We find interesting correlations between the radio spectral index and fesc, ionization conditions, metallicity, SFR density etc. Furthermore, our galaxies show a deviation from the well-studied radio-SFR relation found for normal star-forming galaxies. I will also present an ongoing work on the radio-SED of extremely metal-poor, highly compact starbursts at low-z which show peculiar radio spectral features, providing new insights on their extreme star-formation properties. Overall, our study highlights radio as a unique probe to study the role of supernova feedback, cosmic rays and magnetic fields in facilitating the escape of LyC photons (or their lack) which requires further study both on the observational and simulation front. Our observations will provide guidance for future multi-wavelength studies of high-redshift galaxies and searches for LyC emitters with the current and upcoming highly sensitive facilities like the SKA, ngVLA in synergy with JWST and ELT. Meeting link: https://us02web.zoom.us/j/86088397768?pwd=hGLep2qQPE2AGaEa2N1sw1TX94XDor.1 Meeting ID: 86088397768 Password: 7941

02.10.2025

Michelle Trsirou

Microquasars observed in the gamma-ray band : the parsec-scale outer jets of SS 433 seen with H.E.S.S.

The galactic microquasar SS 433 has been extensively studied across the years, as it presents a unique opportunity to understand jet dynamics thanks to its proximity to Earth, at an estimated distance of ~ 5.5 kpc. The HAWC observatory in 2018 purported very-high-energy (VHE) emission associated to the SS 433 system, indicating that the energetic particles producing the X-ray emission of the outer jets may also be producing VHE gamma rays. The High Energy Stereoscopic System (H.E.S.S.) has discovered, in 2024, emission from the two jets in TeV ranges, revealing an energy-dependent morphology, consistent with X-ray observations. Later results that year from the LHAASO experiment at > 100 TeV, broaden the picture we have of the binary system in the overall gamma-ray band. Thus yielded observables within the high-, very-high-, ultra-high-energy gamma-ray ranges point to a multi-zone model to explain the emission components from this microquasar. In this presentation, I will focus on the SS 433 system and how the spatially-resolved VHE emission highlights a dominant leptonic origin at the multi-TeV energy band. Moreover, indications of the jet flow decelerating abruptly, in addition to evidence of highly efficient acceleration located in the outer jets, suggest the presence of strong shocks at a distance of ~25 pc, either side of the central binary system. Finally I will contextualise our past findings on how they probe this (new) class of gamma-ray sources and how they may enhance our understanding of powerful relativistic jets.

13.10.2025

Dan Hu

Diffuse radio emission within and beyond galaxy clusters

Galaxy clusters are permeated by hot, X-ray–emitting gas extending over megaparsec scales. In addition to this thermal component, many clusters also host diffuse synchrotron radiation, which often correlates spatially with the X-ray emission. This non-thermal emission arises from relativistic electrons (re-)accelerated by merger-driven turbulence or shock fronts within the intracluster medium. Recent discoveries of inter-cluster radio bridges and mega-halos (structures larger than typical radio halos), have suggested the presence of relativistic particles and magnetic fields even in the outskirts of clusters and the connecting large-scale filaments. In this talk, I will provide a brief introduction to diffuse radio emission within and beyond galaxy clusters, and also present our recent MeerKAT discovery of diffuse GHz radio emission extending from Abell 3017 towards Abell 3016.

17.10.2025

Jiří Linda

X-ray spectroscopy of black holes accretion disks - analysis of LMC X-3

In this talk, the main results from the Bachelor Thesis's project will be presented. We provide spectral re-analysis of the X-ray binary system LMC X-3, located in the Large Magellanic Cloud and hosting a stellar-mass black hole, employing the data from the NICER and NuSTAR X-ray telescopes. To analyze the observed energy spectra, we apply relativistic accretion disk models. Our results are consistent with recent studies, which suggest a deviation from the standard thin accretion disk model to a slim disk structure. Given that the precise disk geometry affects X-ray polarization, these findings emphasize the need for further refinement of the slim disk model to incorporate polarization calculations, enabling it to fit the X-ray polarization data from IXPE.

20.11.2025

Alex C. Gormaz-Matamala + Vojta Partik

Joint Journal Club

Joint Journal Club (JJC): Stellar department and Department of Galaxies > Alex C. Gormaz-Matamala will present the paper: "Enhanced Mass Loss of Very Massive Stars: Impact on the Evolution, Binary Processes, and Remnant Mass Spectrum": https://arxiv.org/abs/2505.10206 > Vojta Partik will present the paper: "Citizen Science Identification of Isolated Blue Stellar Systems in the Virgo Cluster" https://ui.adsabs.harvard.edu/abs/2025ApJ...983....2D/abstract

09.12.2025

Marek Abramowicz, Chris Fragile, Gabo Torok, Jiri Horak, Tanja Kaister, Angelos Karakonstantakis, René Sprna, Tom Stanovsky

A not very formal workshop on relativistic astrophysics

An approximate program: 14:00 – Marek Abramowicz (Alpha prescription) 14:30 – Chris Fragile (Lense–Thirring precession of accretion flows in numerical simulations). 15:00 – Coffee break, discussions 15:30 – Gabo Torok (LSD trips) 15:50 – Jiri Horák (Lense–Thirring precession of accretion flows) 16:10 – Gabo Torok (For Whom the Disc Precesses) 16:30 – Coffee break, discussions 17:00 – Tanja Kaister 17:15 – Angelos Karakonstantakis 17:30 – René Sprna 17:45 – Tom Stanovsky 18:00 Discussions