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Seminarium środowe


"Camk Annual Conference"



"CAMK Annual Conference"



"Crusts of accreting neutron stars"

Lami Suleiman (CAMK, Warsaw)

The compression of matter leads to exothermic reactions in the crust of accreting neutron stars. The heat deposited in the crust is transported to the surface and is detectable as a surface luminosity even during quiescence phases. Two common approximations used to model the heat sources of accreting crusts are revised. The first concerns the fully accreted crust approximation: we study a partially accreted crust made of the compressed original crust under freshly accreted material. The second concerns the instantaneous approach to the electron capture kinetics: we show that including the reaction rate of electron captures in the evaluation of deep crust heating leads to an increase in the heat release which can be as high as 20% for the shallowest shells of the outer crust. In collaboration with Prof. J. L. Zdunik, Pro. P. Haensel, Dr. M. Fortin, Mdr. N. Chamel and Dr. A. F. Fantina.


"Doubly eclipsing systems"

Jakub Kolář (Masaryk University Brno)

Doubly eclipsing systems represent a new scientific field in stellar astrophysics. These objects are excellent laboratories for studying the formation mechanisms and evolution of multiple stars. Precise long-term monitoring of photometric data serves well for detecting these systems and brings better knowledge about their numbers and properties. Eclipse time variations can prove the mutual motion of binaries. We would like to introduce our research of observations and modelling of these systems.


"Cyclic Conformal Cosmology"

Krzysztof Meissner (Institute of Theoretical Physics, Warsaw University)

The assumptions behind CCC will be discussed. With the help of Penrose diagrams the connection of our aeon with the previous one will be described. Possible observational traces of the previous aeon will be discussed.


"Simulation of the dynamics and geometry of broad line region in quasars"

Mohamad Naddaf (CAMK, Warsaw)

Broad emission lines are the most characteristic feature in the quasar’s spectra, known since 60 years, but we need a theoretical model for the localisation of the broad line region (BLR), the region that gives rise to production of those broad lines. The most successful non-ad-hoc physically-motivated model during the last decade which consistently explains the formation of broad emission lines in the spectra of quasars and provides the information on the location of BLR is the failed radiatively accelerated dusty outflow (FRADO). The model currently available in the 2.5D advanced version, which I developed, has taken very progressive concrete steps toward the goal. The model is on its way to ultimately pave the path for using quasars in cosmology.


"February 19 - February 21"

Nicolaus Copernicus World Congress (Toruń)

Congress Program:


"Searching for exceptional gravitational-wave sources in the upcoming LIGO/Virgo/KAGRA observing run"

Marek Szczepańczyk (LIGO-Virgo)

Multi-messenger Gravitational-Wave Astrophysics is the new venue for discovery; it is one of the most exciting areas in Science. The gravitational-wave observations by LIGO/Virgo/KAGRA challenge our understanding of the Universe and allow testing theories at an unprecedented level. While predicted, the first binary black hole merger and an intermediate-mass black hole are one of the exceptional sources observed so far that triggered a lot of research on their properties. The gravitational-wave detectors have significantly improved recently, and we expect daily detections in the upcoming 1.5 years' observing run. It's an excellent opportunity for discovery. I will present the preparation to observe new exceptional gravitational-wave sources. I will explain the searches, specifically those in real-time, that are crucial for multi-messenger observations. Finally, I will describe some lessons learned from the previous observing runs and the near-future prospects.


"Galaxy mergers: where are they and what are they good for?"

William Pearson (National Center for Nuclear Research, Warsaw)

Galaxy mergers underpin our current understanding of how the Universe has grown and evolved. As dark matter halos grow hierarchically, the galaxies that they host also merge. These mergers can radically change the morphologies of the interacting galaxies, throwing the material around and create and destroy finer structures. This disruption can also trigger enhanced star-formation events, and enhanced AGN activity. However, exactly when these changes occur, and how powerful they can be, is hotly contested. Thus, large samples of merging galaxies are needed to study these changes in more detail. There are a number of different ways of identifying galaxy mergers; from simply looking at the images to using the latest machine learning techniques. Through this seminar, we will look at different techniques and how we can use them to identify galaxy mergers. We will see how the more advanced techniques will allow us to explore parameter spaces that are often overlooked and find galaxy mergers in surprising ways. This talk will also discuss the ongoing and future directions of merger studies. How we can get around the truth problem: what exactly is a galaxy merger? What kinds of science we can do with merger classifications that we have now and the classifications we will be making in the next few years?


"Probing the baryon cycle of low-metallicity dwarf galaxies"

Nanni Ambra (National Center for Nuclear Research, Warsaw)

The chemical enrichment of galaxies is determined by different physical processes: stellar birth and evolution, dust growth and destruction, large-scale galactic inflows and outflows of material. Understanding the interplay of such processes is essential in order to study the build-up of metals and dust across the cosmic time, and to interpret the available and future observations (e.g. from Spitzer, JWST, ALMA). In this talk, I will review the main uncertainties affecting dust evolution in the interstellar medium of galaxies. I will then present the results of an investigation focused on low-metallicity local dwarf galaxies that takes into account recent observational constrains on their outflow efficiency, as well as other observables, i.e. metallicity, gas fraction, dust-to-stellar mass ratio, specific star formation rate, age of the main stellar population. Such a richness of information allows us to study in details the baryon cycle in these systems. In particular, I will show how the comparison between model predictions with observations allows us to identify the most relevant physical processes driving baryon evolution in local dwarf galaxies. For the first time, we also test the results for different initial mass functions of the stars, i.e. Chabrier and top-heavy.


"Time-dependent modeling of particle acceleration and cooling in blazars"

Anton Dmytriiev (North-West University, South Africa)

Flaring states of blazars are ideally suited to study the extreme physics of relativistic outflows. A thorough understanding of particle acceleration and cooling mechanisms operating in blazar jets can be achieved via physical modeling of varying multi-band flaring emission from radio up to gamma-ray range. The majority of the numerical codes developed for this task use a simplified continuous-loss description for the inverse Compton particle cooling. Such an approximation is however no longer valid in the Klein-Nishina (KN) regime. In our study, we explore the importance of non-continuous Compton cooling losses and their effect on the blazar electron spectrum and broad-band spectral energy distribution (SED) for typical physical conditions during blazar gamma-ray flares. We find that the non-continuous cooling can lead to a significant modification of the shape of the electron spectrum and SED, especially during flaring states of FSRQs characterized by high Compton dominance.


"Proto-neutron star evolution and neutrino interactions in hot and dense matter"

Micaela Oertel (LUTH, Meudon)

Neutrinos play an important role in compact star astrophysics: neutrino-heating is one of the main ingredients in core-collapse supernovae, neutrino-matter interactions determine the composition of matter in binary neutron star mergers and have among others a strong impact on conditions for heavy element nucleosynthesis and neutron star cooling is dominated by neutrino emission except for very old stars. Many works in the last decades have shown that in dense matter medium effects considerably change the neutrino-matter interaction rates, whereas many astrophysical simulations use analytic approximations which are often far from reproducing more complete calculations. In this talk I will present a scheme which allows to incorporate improved rates into simulations and show as an example results for the evolution of a proto-neutron star.


"Evolution of massive stars with new hydrodynamic wind models"

Alex Gormaz-Matamala (CAMK, Warsaw)

We present evolutionary models for a set of massive stars, introducing a new prescription for the mass loss rate obtained from hydrodynamic calculations in which the wind velocity profile and the line-acceleration are obtained in a self consistently way. Evolutionary models with the new recipe for mass loss retain more stellar mass through their evolution, which is expressed in larger radii and consequently more luminous tracks over the Hertzsprung-Russell diagram. Also, models with self-consistent winds predict a weaker braking in the rotational velocity and a more marked drift redwards of the evolutionary tracks across the HRD, as a direct consequence of the differences in the stellar angular momentum loss and in the rotational mixing. Together with the prediction of higher masses at the end of main sequence, self-consistent tracks also predicts a distribution of rotational velocities for Galactic O-type stars more in agreement with the diagnostics of recent surveys. Other hypothetical implications, such as the masses of Ofpe stars at the Galactic Centre or the contribution of the isotope Al-26 to the ISM, are open to discussion.


"The chemodynamical nature of the stellar overdensities in the Milky Way"

Hélio Perottoni (University of São Paulo, Brazil and CAMK, Warsaw)

Over the past two decades, several stellar structures have been discovered both near the Galactic plane and in the halo. Some of the most intriguing of these structures are stellar overdensities, which are tenuous and spatially extended clouds of stars. Those stellar overdensities are identified as stellar count excesses in a given region of the sky when compared to other Galactic halo fields. Their nature has been debated since their discovery between an in situ and an ex-situ origin, although it seems that a consensus has finally been reached. In spite of this, there are still open questions regarding their characterization and how and when they formed. In this talk, I will discuss the nature of halo stellar overdensities and the characterization of disk stellar overdensities. I will present (i) a historical overview of these structures; (ii) the results from chemodynamical analyses our group performed for several of them.


"Carbon, oxygen, and iron abundances with 3D non-LTE models"

Anish Amarsi (Department of Physics and Astronomy, Uppsala University)

Spectroscopic analyses of stellar chemical compositions are model-dependent, and shortcomings in the models often limit the accuracy of the final results. For late-type stars like our Sun, two of the main problems in present-day methods are that they assume the stellar atmosphere is a) one-dimensional (1D) and hydrostatic, and b) satisfies local thermodynamic equilibrium (LTE). We can relax these assumptions simultaneously by performing detailed 3D non-LTE radiative transfer post-processing of 3D radiative-hydrodynamic model stellar atmospheres. I shall briefly describe this approach, and then illustrate its impact on carbon, oxygen, and iron abundances in late-type stars, and thereby on our understanding of stars and of the Galaxy.


"Combining binary and asteroseismic information in stellar modelling"

Cole Johnston (Radboud University, Nijmegen, NL)

Large-scale high-precision space-based photometric missions have revealed that most stars are variable either through intrinsic (spots, pulsations) or external (eclipses) mechanisms. Fortunately, we can leverage the characteristics of this variability in order to learn something about stars. For instance, spots provide information on stellar rotation, pulsations reveal the interior structure of stars, and eclipses provide the opportunity to obtain highly precise estimates of the fundamental stellar parameters. Fortuitously, it is clear that many stars display multiple forms of variability. To this end, I will discuss my work in jointly modelling pulsating and eclipsing binary variables to cross-calibrate stellar evolutionary models at all masses across the Hertzsprung-Russell diagram.


"Research at AstroCeNT in the context of the Global Argon Dark Matter program"

Marcin Kuźniak (CAMK, AstroCeNT, Warsaw)

The search for interactions of dark matter particles (WIMPs) with ordinary matter is carried out with large detectors, with currently the most promising detection technology based on the use of a large mass of liquid argon or xenon as the detector medium. Efficient collection and detection of light from multi-tonne detectors poses challenges and motivates dedicated R&D on optical materials, including wavelength shifters, as well as on photosensors. Such work at AstroCeNT, either in the context of the Global Argon Dark Matter Collaboration, or general R&D applicable also to neutrino detectors, will be presented, together with an overview of the field.


"Internal kinematics of the Milky Way dSph satellites: observations and simulations"

Alberto Martinez-Garcia (Instituto de Astrofísica de Canarias)

Dwarf spheroidal galaxies (dSph) are the most numerous satellites of the Milky Way (MW). Due to their proximity, their stellar population can be resolved, allowing us to study them in depth. However, the internal kinematics of dSphs remains one of their least known aspects. We present a kinematic study of six of the dSph satellites of the MW. We combine proper motions from the Gaia DR3 and line-of-sight velocities from previous works to derive their 3D internal kinematics. We detect coherent motions in a number of these systems. We find, for the first time, rotation in the plane of the sky in three of the classical satellites of the MW, namely Carina, Fornax and Sculptor. We also detect velocity gradients in Draco, Ursa Minor, Carina, and Fornax. The amplitude of the gradients appears to be related to the orbital history of the dSphs, suggesting that the interaction with the MW is causing them. We also explore the internal kinematics of dwarf satellites of the TNG50 simulation to asses our observational findings in the MW and the role that the interaction with the host plays in the evolution of the internal kinematics.


"The Important Role of Cosmic Ray Transport in Galaxy Evolution"

Philipp Kempski (Princeton University)

Although negligible by number density, relativistic cosmic rays (CRs) are an energetically important component of the plasmas filling galaxies and galaxy clusters. As a result, these particles may play an important role in shaping large-scale galactic (thermo)dynamics, for example by driving galactic winds and/or heating diffuse gas in galaxy halos. For this reason, “CR feedback” has become a key ingredient in galaxy evolution models. However, a fundamental limitation of these models is that the nature of CR feedback is a very strong function of the assumed CR transport, which remains uncertain. In this talk, I will give an overview of CR feedback in galaxies, discuss theoretical uncertainties in CR propagation and how CR spectra measured at Earth help us constrain the physics of their transport. I will argue that popular existing transport models are full of theoretical uncertainties and are generally not in good agreement with observations. This suggests that we may need a new theory of CR transport.


"Overview of Dark Matter Production in Nonstandard Cosmologies with a Focus on Axions"

Jacek Ksawery Osinski (AstroCent/CAMK, Warsaw)

The Universe is typically assumed to be radiation dominated in the period preceding big bang nucleosynthesis, however we do not currently have any observational probes to confirm this. Nonstandard cosmological histories, where some other form of energy density dominates for a time, commonly arise in theories of the early Universe. These histories have important consequences for processes, such as dark matter production, occurring in that time. In this talk I will briefly review nonstandard cosmological histories and their effects on dark matter production focusing on axion dark matter.


"White Dwarf Pulsars"

David Buckley (SAAO/SALT, University of Cape Town, University of the Free State)

The 2016 discovery of the unusual nature of the detached White Dwarf – Red Dwarf binary, AR Scorpii, has led to efforts to discover more examples of such systems. In this talk I will review the multi-wavelength properties of AR Sco, a system whose luminosity is dominated by non-thermal strongly polarized emission, pulsed at the ~ 2 min spin and beat periods. In many respects the system shares characteristics of a pulsar and is powered by the spin-down of the White Dwarf. I will also present preliminary results of a simultaneous multi-wavelength campaign on AR Sco, conducted in June 2020, involved optical observations (time resolved spectroscopy and photometry) from SALT, Keck and the SAAO 1-m telescope. In addition, X-ray observations were obtained with NICER and L-band radio observations with the MeerKAT radio telescope array. These observations are used to investigate the wavelength dependence of the periodic modulations seen in AR Sco, across the E-M spectrum. Finally, I will present observational results of a second AR Sco-like system, J191213.72−441045.1, discovered in 2022, which shares many similar characteristics with AR Sco, including pulsed X-ray, optical and radio emission and is also strongly polarized.


"Copernicus’s Mathematical Astronomy in "De revolutionibus": A Very Short Introduction"

Jarosław Włodarczyk (Institute for the History of Science, PAN, Warsaw)

Nicolaus Copernicus (1473–1543) propelled one of the greatest revolution in science and his heliocentric cosmology radically changed the relationship between man and the universe. However, to prove his concept of cosmos, Copernicus had to create mathematical models of the movements of planets. He devoted five of the six books of his "De revolutionibus" (On the Revolutions, 1543) to this task. As he himself argued, “mathematics is written for mathematicians”. In my lecture I shall outline Copernicus’s mathematical astronomy and its early reception in the form of the first Copernican astronomical tables which allowed to calculate the positions of the planets for any time.


"Testing cosmological models with cross-correlation measurements"

Chandra Shekhar Saraf (CAMK, Warsaw)

The well established standard ΛCDM model of cosmology suffers from several challenges and disparities like the nature of dark matter and dark energy, accelerated expansion of the Universe and tensions in cosmological parameters derived from different observations. These shortcomings have raised questions on the validity of the ΛCDM model and led to increased interests towards alternative models of cosmology (dark energy models and Modified Gravity theories). Cross-correlation measurements at different redshift bins from the next-generation CMB experiments and galaxy surveys will provide us an unprecedented opportunity to put stronger constraints on alternative models of cosmology. However, in order to accurately quantify deviations from the ΛCDM model, it is crucial to mitigate the systematic errors that may bias the cross-correlation measurements. In this presentation, we will talk about various systematics that can impact the estimation of cosmological parameters in cross-correlation studies. We will present results from the first cross-correlation measurements between Planck CMB lensing potential and the Herschel Extragalactic Legacy Project (HELP) galaxy catalogues. In addition, we will also discuss the implications of leakage of objects in tomographic cross-correlation studies, through simulations performed with expectations from the upcoming LSST survey.


"Breaking degeneracies between models using X-ray polarisation observations"

Yash Bhargava (Tata Institute for Fundamental Research, Bombay, India)

Observations of the accreting compact objects have been paramount in constraining the physical processes in extreme gravity regimes. In particular, the X-ray observations probe the spacetimes closest to the compact object and often embed the signatures of the geometry of these systems. Various persistent and transient X-ray binaries have been regularly observed using a slew of space missions and thus we now have some key insights into these enigmatic objects. Recently, there have been major headways in probing the polarisation of these objects in X-rays, due to the launch of IXPE and AstroSat. In this talk I will present some of the recent work done by us to investigate the polarisation of various X-ray binaries and how these observations have broken the degeneracies which are impossible to break using the standard spectral and timing studies.


"Stellar and supermassive black holes: similarities and differences"

Chris Done (University of Durham)

I will review what we know about accretion physics in black hole binaries and then scale it up to the supermassive black holes in AGN and quasars to see what we can learn from the similarities and especially the differences between them.

This is a special edition of Wednesday Colloquium and will be heald on Monday, June 17th, at 11:15 am in a lecture hall.


"Complex evolutionary and asteroseismic modelling of pulsating components in eclipsing binary systems"

Amadeusz Miszuda (CAMK, Warsaw)

Eclipsing binary systems are well-proven benchmarks in testing stellar evolution theory. Precise stellar parameters that can come from their analysis aid systems' age determination along with tracing back their evolution history. Eclipsing binaries that contain pulsating components are a special subclass of binaries that combine information coming from orbital and pulsational analyses. Whereas the single-star pulsators are rather well understood, the precise effect that binarity and possible mass transfer have on the pulsational characteristics of components has yet to be determined. In my talk, I will highlight some of the most important results from my studies of pulsating components in binary systems. In particular I will discuss the effect that accumulation of He in outer layers of accretor caused by mass transfer has on excitation of the high radial-order g modes in models of delta Scuti stars and the evolution of radial mode frequencies during the main sequence evolutionary phase.


"Magnetic white dwarfs in close binaries: origin, evolution, surfeit, and dearth"

Diogo Belloni (Departamento de Física, Universidad Técnica Federico Santa María)

Since the 1940s, understanding why some white dwarfs become strongly magnetized while others do not has been one of the fundamental unsolved issues of stellar evolution. Several models have been suggested for the magnetic field generation in white dwarfs, but they all fail when confronted with the observed incidence of magnetism in single white dwarfs and white dwarfs in binaries. We recently suggested that a rotation- and crystallization-driven dynamo, similar to those operating in planets and low-mass stars, is responsible for a large fraction of the observed strongly magnetized white dwarfs. During this talk, I will show that this new scenario can explain the existence of pulsar-like white dwarfs, the absence of bright X-ray accreting white dwarfs in globular clusters, the accumulation of magnetic white dwarfs among metal-polluted white dwarfs, the occurrence rate of strong magnetic fields in double white dwarf binaries, the paucity of white dwarfs accreting from brown dwarfs, and why a large fraction of accreting white dwarfs but only a small part of their detached progenitors contain a strongly magnetized white dwarf. That implies a rotation- and crystallization-driven dynamo most likely plays an unprecedentedly important role in the magnetic field generation in white dwarfs.


"Dynamical evolution of the Milky Way globular clusters in cosmological timescale"

Marina Ishchenko (Main Astronomical Observatory, National Academy of Sciences of Ukraine)

The main idea of the work is to perform the dynamic evolution of the orbits of Globular Cluster subsystems (GCs) with a look-back time up to 10 Gyr. This will allow us to estimate the possible interaction of GCs with the Galactic Center (GalC) region, including the influence of the super-massive black hole (SMBH), which has dynamically changed in the past and collisional rates with each other’s (GC with GC). To reproduce the structure of the Galaxy in time, we used external potentials selected from the large-scale cosmological database IllustrisTNG-100, whose properties (mass and size of the disk and halo) are similar to the physical values of the Milky Way at present. In these potentials, we have reproduced the orbits of 159 GCs in 10 Gyr lookback time using our own high-order N-body parallel dynamical code, the phi-GPU code. For the initial proper motions, radial velocity and heliocentric distance of each GC, we take prom from the Baumgardt & Vasiliev (2021) catalog based on Gaia Data Release 3. To identify clusters that have interaction with the GalC we used the criteria that the relative distance between the SMBH and the GC is at least four times the sums of the GCs half-mass radii. Using these simple criteria, we obtained statistically significant rates of close passages of the GCs with the Galactic Center. During our investigation, we analyzed the GC orbital evolution to find a GC that has such a close passage and prepare the statistical probability of such events. For the selected GCs, we generated an initial mass function and performed a full N-body modeling to find out the potential influence on the GCs stellar populations due to the SMBH influence in the Galactic Center region. Also, we present a statistical analysis of the cumulative close passages rate: about ten close passages with relative distances shorter than 50 pc for every billion years for each of the five external potentials. We present the 22 most reliable collision pairs with a good probability. As an example: Terzan 4 versus Terzan 2, Terzan 4 versus NGC 6624, Terzan 4 versus Terzan 5, Terzan 4 versus NGC 6440, and Terzan 4 versus Liller 1. The most active globular cluster in the collision sense is Terzan 4, which has 5.65 collision events on average. Most collisions are located inside the Galactic disk and form two ring-like structures. The first ring-like structure has the highest collision number density at 1 kpc, and the second structure has a maximum at 2 kpc.


"Surveying the skies at high sensitivity: The anticipated science with the Rubin Observatory / LSST"

Greg Madejski (Kavli Institute for Particle Astrophysics and Cosmology, SLAC and Stanford University)

Vera Rubin Observatory is under construction at the Cero Pachon mountain summit in Chile. The observatory will feature a mirror with 8 meter diameter, and the largest CCD camera in its focal plane, with 3.2 gigapixels. The large field of view of the telescope - 9.6 square degrees - will allow for efficient surveys of large fraction of the sky. During the 10-year the "Legacy Survey of Space and Time" - LSST - the observatory will study a wide range of celestial phenomena. This includes making a sensitive measurement of the evolution of dark energy as a function of cosmic time, study of transient celestial sources such as supernovae, and taking census of small bodies in the Solar System. This presentation will describe those scientific goals in some detail, and will present the current status of the project.


"Auroras on planets around pulsars"

Miljenko Čemeljić (CAMK, Warsaw)

In analogy with the Solar system planets, we predict the existence of aurora on planets around millisecond pulsars. From our magnetohydrodynamic (MHD) simulations of magnetospheric pulsar-planet interaction, we estimate the radio emission from such systems. We find that the radio emission from aurora on pulsar planets should be observable with the current instruments, and provide parameters for such a detection. It would be the first radio detection of an extrasolar planet and the first direct probe in the pulsar wind.


"Shedding Light on Low Surface Brightness Galaxies in Dark Energy Survey with Transformers"

Hareesh Thuruthipilly (National Center for Nuclear Research, Warsaw)

Low surface brightness galaxies (LSBGs) which are defined as galaxies that are fainter than the night sky, play a crucial role in understanding galaxy evolution and cosmological models. Upcoming large-scale surveys like Rubin Observatory Legacy Survey of Space and Time (LSST) and Euclid are expected to observe billions of astronomical objects. In this context, using semi-automatic methods to identify LSBGs would be a highly challenging and time-consuming process and demand automated or machine learning-based methods to overcome this challenge. We study the use of transformer models in separating LSBGs from artefacts in the data from the Dark Energy Survey (DES) data release 1. Using the transformer models, we search for new LSBGs from the DES that the previous searches may have missed. We identified 4,083 new LSBGs in DES, adding an additional 17% to the LSBGs already known in DES. This also increased the number density of LSBGs in DES to 5.5 deg^{-2}. Properties of the newly found LSBGs are investigated, along with an analysis of the properties of the total LSBG sample in DES.


"The challenges of the modeling Changing Look Active Nuclei and nuclear transients phenomena"

Marzena Śniegowska (Tel Aviv University and CAMK PAN)

Apart from regular, low-level stochastic variability, some AGNs occasionally show exceptionally large changes in luminosity, spectral shape, and/or X-ray absorption that occur on surprisingly short timescales (even several weeks). The most notable are the changes of the spectral type when the source classified as a Seyfert 1 becomes a Seyfert 2 galaxy or vice versa. Thus a name was coined as 'Changing-Look AGN' (CL AGN). The origin of this phenomenon is still unknown, but for most of the sources, there are strong arguments in favor of intrinsic changes. Understanding the nature of such rapid changes is a challenge to the models of black hole accretion flows since the timescales of the changes are much shorter than the standard disk viscous timescale, related to changes in angular momentum distribution. I will present recent results regarding the model of the time-dependent evolution of a black hole accretion disk unstable due to the dominant radiation pressure as the CL AGN phenomenon scenario. I will discuss also poorly understood flaring AGN with broad Bowen fluorescence emission features, driven by extreme UV radiation that appears within weeks but lasts for well over a year. Those events observationally differ from the tidal disruption events known to date, however, the physics behind them may be interlinked.


"The extremes of AGN variability: changing-look AGN and supermassive binary black holes"

Stefanie Komossa (Max-Planck-Institute for Radio Astronomy, Bonn)

Highly variable and transient active galactic nuclei (AGN) provide us with important insights into the physics of accretion of matter onto their central supermassive black holes (SMBHs). This talk discusses some of the most extreme cases of variability so far observed among AGN, including the highest-amplitude X-ray--optical outbursts, deep-minima states, and exceptional optical emission-line changes, and their interpretation, based on observations taken at mutiple wavebands from the radio to the X-ray regime. A fraction of AGN harbor binary SMBHs at their centers. Upon final coalescence, these are the loudest sources of gravitational waves in the universe. After a brief introduction of the topic, I will then discuss the best-known case of a compact binary SMBH candidate, hosted by the blazar OJ 287. In the course of the project MOMO, we have carried out the densest, longest, multi-wavelength monitoring project of OJ 287 done so far from radio to high energies, and I will highlight key results and implications for binary SMBH scenarios.


"Accretion structures around Reissner-Nordstrom naked singularity and Sgr A* constraints"

Ruchi Mishra (CAMK, Warsaw)

Naked singularities are horizon less compact objects in General Relativity. Here We focus on the simplest scenario of Reissner-Nordstrom (RN) space-time which corresponds to a naked singularity for Q>M. A general property of this metric is the existence of a radius, known as the zero-gravity radius, where a test particle would remain at rest. Outside this radius gravity is attractive, inside of it gravity is repulsive. We focus on the accretion process and construct polytropic tori with uniform values of specific angular momentum in the naked singularity regime of the RN metric. Furthermore, assuming the torus in the compact source Sgr A* at the Galactic center observed by the Event Horizon Telescope (EHT) to be the image of a quasi-stationary fluid structure, we provide rigorous constraints in the RN metric on the electric charge-to-mass ratio Q/M of Sgr A*.


"Linear and non-linear statistics of cosmic density fields in modified gravity cosmologies"

Suhani Gupta (Center for Theoretical Physics Polish Academy of Sciences, Warsaw)


"What was the last Nobel Prize in Physics given for?"

Katarzyna Krajewska (Institute for Theoretical Physics, Warsaw University)