Wednesday Colloquium


"Towards accurate calibration of the cosmic distance scale: Pulsating stars in the solar neighbourhood in the Gaia era"

Piotr Wielgórski (CAMK, Warsaw)

Launched in 2013, the Gaia space mission provides parallaxes of billions of stars in our neighborhood with unprecedented accuracy. I will present recent results of the Araucaria project where we use Gaia parallaxes of nearby pulsating stars and data obtained in the Cerro Armazones Observatory to improve the accuracy of distance measurements in the Universe.


"26-27 January"

CAMK Annual Conference (CAMK, Warsaw)


"Stability-Causality Theorems"

Lorenzo Gavassino (CAMK, Warsaw)

Relativistic hydrodynamics is tricky. There are pitfalls at every corner. Many authoritative textbooks, which are currently used as standard references both in physics and astronomy, present several hydrodynamic equations that lead to completely unphysical predictions (e.g., a glass of water should spontaneously detonate in 10^-34 seconds!). Among such books, we find Novikov-Thorne ("Astrophysics of black holes"), Misner-Thorne-Wheeler ("Gravitation"), Mihalas-Mihalas ("Foundations of radiation hydrodynamics"), and many others. Hence, it is not surprising that such incorrect equations have become part of the "common sense" of a considerable fraction of the astrophysical community, being used both in theoretical models and numerical simulations. How bad is the situation? For decades, some of the brightest minds (e.g. Carter, Israel, Lindblom, Geroch, Anile, Ruggeri) have been looking for more reliable equations, but nobody ever analyzed the problem in full generality. What makes a theory problematic? What makes a theory reliable? What happens if we simulate the wrong theory? Is there an intuitive explanation for all of this? I will provide the definitive answer to all these questions, and I will do it without writing a single equation.


"A multitude of Multiverses: what is "Multiverse"?"

Łukasz Lamża (Copernicus Center for Interdisciplinary Studies, Jagiellonian University)

The concept of "Multiverse" enjoys varying popularity in contemporary cosmology, with some physicists considering it a viable explanation of cosmological facts, and other denying it any claims for scientific nature. Here, a simple typology of Multiverses is presented to help in organizing and understanding the wealth of studies on the subject: 1) non-cosmological Multiverse (e.g. Everett's many-worlds interpretation of quantum mechanics, Susskind's string theory landscape); 2) cosmological, mathematical Multiverse (e.g. Carter's ensemble of Universes, Tegmark's level 4 Multiverse); 3) cosmological, physical Multiverse (e.g. Tolman's cyclic model, Linde's chaotic inflationary multiverse, Smolin's cosmological natural selection, Penrose's conformal cyclic cosmology). Their theoretical and observational status is shortly summarized.


"Magnetic field in neutron stars: from the interior to the exterior"

Ankan Sur (CAMK, Warsaw)

The origin of the magnetic field in neutron stars, which has strengths trillions of times stronger than terrestrial magnets, remains a mystery to date. To unravel this mystery, modeling the magnetic field and understanding its equilibrium are critical. A key step towards this understanding is magnetohydrodynamics (MHD) studies. In this talk, I will discuss the results of our MHD simulations from which we had obtained various magnetic field configurations. While these results come from time-evolving systems, I will also discuss equilibrium solutions from the Grad-Shafranov equation for a normal matter crust and superconducting core neutron star. These results are applicable for the standard pulsar population. Modeling the emission from neutron stars, such as gravitational waves or electromagnetic waves, is another important step towards understanding the magnetic field. As an example, I will show how a newly millisecond magnetar may form accretion "mountains" and emit gravitational waves. And lastly, I will also discuss how the geometry of the magnetic field in pulsars can be constrained based on radio observations.


"Quantum principle of relativity"

Andrzej Dragan (Institute of Theoretical Physics, Warsaw University and National University of Singapore)

We show that the local and deterministic mode of description is not only in conflict with the quantum theory, but also with relativity. We argue that elementary relativistic properties of spacetime lead to the emergence of a non-deterministic quantum-mechanical picture involving quantum superpositions and complex probability amplitudes.


"GWTC-3 and the origin of gravitational wave sources"

Tomasz Bulik (CAMK, AstroCent, UW, Warsaw)

The recent catalogue of gravitational wave sources GWTC-3 provided a wealth of information on the properties of merging binaries that already allow some statistical studies. I will summarize these information. I will then proceed to models of formation of these objects and present their properties. Finally, I will confront the two and discuss the viable options for formation of merging binaries observed in gravitational waves.


"Using classical Cepheids to study the far side of the Milky Way disk"

Javier Minniti (CAMK, Warsaw)

Classical Cepheids (CCs) are among the most useful Galactic and nearby extragalactic distance tracers because of their well-defined period-luminosity relations. Moreover they are young and luminous stars, with characteristic light variations that make them - relatively - easy identifiable. For these reasons, they are excellent standard candles and ideal tracers of the Galactic disk. Their location at the Milky Way disk complicates their identification, mainly due to the substantial reddening they are subject to. This can be surpassed through the use of infrared (IR) photometry. Using data from the Vista Variables in the Vía Láctea (VVV) Survey we are now able to study these young standard candles in highly reddened regions of our Galaxy that were previously hidden to us. However, the classification based solely on near-IR light curves has proven difficult, and prone to providing highly contaminated samples. In this talk, I will show how the use of additional observable properties aids in the light-curve based classification process and present two different approaches to obtain clean samples of CCs: (1) Using spectroscopic follow-up data for a sample of CC candidates. (2) Using proper motion information obtained from the VVV survey. We have been able to significantly increase the number of bona fide CCs at the far disk using near-IR photometric and spectroscopic data. The obtained samples will be used to characterize the properties of the far Galactic disk.


"Global radiative 3D GRMHD models of geometrically thin strongly magnetized disks"

Bhupendra Mishra (Los Alamos National Lab, New Mexico US)

The radiation pressure supported standard accretion disk model (Shakura Sunyaev 1973 model) is prone to classical thermal and viscous instabilities. However, these instabilities are not observed in astrophysical systems as disks remain thermally stable for astronomical time periods. In this talk, I will show some of the key findings of strongly magnetized accretion disks simulated using 3D global radiative GRMHD simulations. We used a set of initial magnetic field configurations to produce an enhanced magnetic field amplification and hence stabilize the radiation pressure dominated accretion flows around stellar mass black holes. We also found that despite the magnetic pressure support, the luminosity and mass accretion rates do match with what is expected from a standard accretion disk model.

The Colloquium will start at 4:15 pm


"Constraining the Cosmology with Quasars"

Raj Prince (Center for Theoretical Physics, Polish Academy of Sciences, Warsaw, Poland)

Quantifying the accelerated expansion of the Universe is one of the key issues of cosmology. Various probes are used for this purpose, like observations of the Cosmic Microwave Background, Supernovae Ia, Baryon Acoustic Oscillations, gravitational lensing, and gamma-ray bursts. Quasars, or more generally, Active Galactic Nuclei (AGN) also joined the class of sources with cosmological applications, and several specific methods to use these objects were proposed: continuum time delays and emission-line time delays. Several recent measurements based on different methods imply the tension between the Hubble constant H0 determination based on the early Universe and the value coming from the relatively local measurements-SNIa has posed many serious questions on the standard cosmological model. I will talk about one of the local measurements using the quasars and how we can use them as a cosmological tool. While addressing the quasars, I will also discuss the issues that we should be careful about in their application to cosmology.


"Coevolution of Supermassive Black Holes and Host Galaxies on cosmological time scales"

Peter Berczik (Main Astronomical Observatory, National Academy of Sciences of Ukraine)

Supermassive black holes are unique objects: the physical description, the origin and evolution of which (i.e., the so-called “life cycle”) and is one of the biggest problems in modern astrophysics. Determining the masses and sizes of black holes at different redshifts, as well as finding double AGNs, are extremely important. Such binary systems are one of the most high-energy dynamic objects in the centers of galaxies. Simulations of a dusty torus for the different initial conditions taking into account the effects which are present in the AGNs central regions will allow to explain a number of observations in the different wave bands. On a base the machine learning algorithms the complex link between spectroscopic and photometric data of modern surveys will been establish. The obtained regularities will allow to create the most complete catalogue of extragalactic sources and to carry out the classification and to obtain additional information from the largest modern surveys of the sky.

Seminar in hybrid version.


"On halo and galaxy connection with the cosmic web"

Wojciech Hellwing (Center for Theoretical Physics, Warsaw)

The Cosmic Web -- an intricate network of clusters, filaments, walls, and walls -- can have many aspects. Most of the features of the Web and the associated impact they induce on the formation and evolution of haloes and galaxies are subject to ongoing debate. The differences are mostly driven by a specific choice among many existing-working definitions of the Cosmic Web. Nonetheless, it is clear that different segments of this net establish different intimate environments and ecosystems for the local halo and galaxy formation. In this talk, I will demonstrate, using the NEXUS+ definition for the Cosmic Web identification, how indeed the disparate environments affect the local formation histories and hence properties of dark matter clumps and galaxies living within. I will focus on internal kinematics and morphological features and distribution of satellite systems. Our findings indicate a fundamental role that the local cosmic environment plays in shaping galaxy and halo formation histories, which in turn have profound implications for their intrinsic properties.


"AGN fueling and feedback: role of molecular tori"

Francoise Combes (College de France and Observatoire de Paris)

Gas inflows directly fueling AGN are now traceable with current high-resolution observations with ALMA and NOEMA. Dynamical mechanisms are essential to exchange angular momentum and drive the gas to the super-massive black hole. While at 100pc scale, the gas is sometimes stalled in nuclear rings, recent observations reaching ~10pc scale (or 50mas), inside the sphere of influence of the black hole, may bring smoking gun evidence of fueling, within a randomly oriented nuclear molecular disk. AGN feedback as molecular outflows are also observed at high resolution helping to identify the responsible mechanisms, either radiative of kinetic AGN mode, or starburst.


(Research Associate, Cardiff University)

Research Associate, Cardiff University


"Studying Magnetic-driven Outflows from Magnetized Neutrino-cooled Accretion Disks"

Fatemeh Hossein-Nouri (Center for Theoretical Physics, Warsaw)

Neutrino-cooled accretion flow around a spinning black hole, produced by a black hole-neutron star (BHNS) or a binary neutron star (BNS) merger is a promising scenario for jet formation and magnetic-driven outflows. Based on GW170817 gravitational wave detection by LIGO and Virgo observatories followed by electromagnetic counterparts, this model can explain the central engine of the short duration gamma ray bursts (GRB) and kilonova radiations. Using the open-source GRMHD HARM_COOL code, we evolve several magnetized accretion disk-black hole models with realistic equation of state in the fixed curved space-time background. The disk and black hole’s initial parameters are chosen in a way to represent different possible post-merger scenarios of the merging compact objects. We identify the effects of disk’s mass and black hole’s spin on the disk’s evolution, paying particular attention to measuring the properties of the ejected outflows. These results are used to estimate the luminosity and light curves of possible radioactively powered transients emitted by such systems.


"Dynamical double black holes in star clusters"

Debatri Chattopadhyay (Cardiff University)

In this talk, I will present an investigation on the relationship between the global properties of star clusters and their double black hole (DBH) populations. For this study, the code NBODY6 is used to evolve a suite of star cluster models with an initial mass of ~O(1e4)Msun and varying initial parameters. This work concluded that cluster metallicity plays the most significant role in determining the lifespan of a cluster, while the initial half-mass radius is dominant in setting the rate of BH exchange interactions in the central cluster regions. It is also observed that the mass of interacting BHs, rather than how frequently their interactions with other BHs occur, is more crucial in the thermal expansion and eventual evaporation of the cluster. We formulate a novel approach to easily quantify the degree of BH-BH dynamical activity in each model. 12 in-cluster and three out-of-cluster (after ejection from the cluster) DBH mergers of different types across the eleven models are presented. It is noted that cluster initial density plays the most crucial role in determining the number of DBH mergers, with the potential hint of too-high stellar density preventing in-cluster BH mergers.


"Variability and evolution of Active Galactic Nuclei"

Marzena Śniegowska (CAMK, Warsaw)

The time-dependent phenomena in Active Galactic Nuclei (AGN) are not yet well-understood. Those sources are massive, with a broad range of physical properties and observations across the entire electromagnetic spectrum to explore. I explore different timescales, from hours/days to the chemical evolution of the galaxy over millions of years. Short-term variations are observed in the form of Quasi-Periodic Eruptions (QPE) and changing look (CL) active galactic nuclei - CL AGN. I discuss the model of radiation pressure instability taking into account the presence of inner advection-dominated accretion flow (ADAF) or the presence of the magnetic field. The long-term evolution shows through the correlation between the observed accretion rate and the chemical composition. I address this issue by determining the metallicity of high accretion rate sources.


"Neutron stars: thermodynamics and hydrodynamics"

Giovanni Camelio (CAMK, Warsaw)

In this seminar I will present the research I carried on in CAMK in the last two years. First, I will talk about the modeling of the binary neutron star merger remnant. If the remnant is not massive enough to immediately collapse to a black hole, after approximately 30 ms from the merger it can be described as a stationary neutron star in differential rotation and with a hot ring on the equatorial plane. This configuration is nonbarotropic, namely the thermodynamic variables cannot be put in a one-to-one relationship. We developed a new method to solve the force balance equation for nonbarotropic stars and performed an extensive parameter study of the remnant. Then, I will talk about bulk viscosity in neutron star codes. Bulk viscosity is a dissipative process that occurs in out-of-equilibrium systems, as for example a multi-component fluid with reacting particle species. We developed a new one-dimensional general relativistic hydrodynamic code for comparing different approaches to bulk viscosity in neutron stars. In particular, we consider the `exact' approach of tracking the different particle species of the multi-component fluid, and two Mueller-Israel-Stewart models where bulk viscosity is approximately included in the stress energy tensor with the bulk stress. We found that the Mueller-Israel-Stewart approaches are good approximation but that the multi-component fluid is easier to implement and more accurate.


"Disk-Planet Interactions: Formation of Mean-motion Resonances in a Gaseous Protoplanetary Disk"

Zijia Cui (University of Szczecin)

In recent decades, more than 800 multi-planetary systems with a great variety of the architectures have been discovered. The fundamental role in shaping the planetary systems during the early stages of their evolution, plays planetary migration. The newly born or still forming planets embedded in a gaseous protoplanetary disk move around their host stars in orbits modified continuously by their gravitational interactions with the material in the disk. It is expected that, when the relative migration of two planets is convergent, then the capture into a mean-motion resonance can occur. Is this expectation always met? In my presentation I will answer this question, on the basis of a full two-dimensional hydrodynamic treatment of the disk-planet interactions, accompanied, where relevant, with the analytic estimates of the effects of the density waves excited by the planets in the disk. I will focus on the systems containing the most numerous planets known till now, called super-Earths or mini-Neptunes. I will demonstrate that a system of two super-Earths can be repelled from the first-order resonance configurations due to the wave-planet interactions and I will give the conditions, which must be satisfied for that. My results provide one of the possible reasons why there are not so many planet pairs observed to be locked in the mean-motion resonances.



After June 8th summer break till end of September

"First image of the black hole shadow in the Galactic Center"

Maciej Wielgus (Max Planck Institute for Radio Astronomy, Bonn,)

There is a supermassive compact object, named Sagittarius A*, in the center of the Milky Way. We have recently obtained the highest resolution images of this object with the Event Horizon Telescope, a global network of radio telescopes operating at high frequency of 230 GHz. In this talk I will discuss the challenges related to obtaining these images, their theoretical interpretation, as well as the question of how much confidence do we have in Sagittarius A* being a Kerr black hole, following these new results.



Since June 9th - summer break till end of September


"Simulating globular cluster populations around Milky Way and Andromeda-like galaxies with MOCCA models"

Agostino Leveque (CAMK, PAN)

The ability to model both the large-scale dynamics (0.01-10 kpc) controlling globular clusters (GC) orbital evolution, and the small-scale dynamics (sub-pc-AU) controlling the internal GC dynamics are essential for a detailed investigation of the co-evolution of GC systems (GCS) in galaxies. We introduced a novel method for the simultaneous evolution of large GCS that combines fully self-consistent Monte Carlo MOCCA models with semi-analytic GCS models. In order to reproduce the observable features of GCs in the Milky Way and Andromeda, we generated synthetic GC populations using the population synthesis code MASinGa and the MOCCA-Survey Database I. In this study, an attempt was made to determine constraints to the spatial distribution of GCs in both galaxies and the contribution of GC to the formation of the nuclear star cluster and supermassive black holes. Additionally, the black hole and black hole-black hole binaries populations in globular clusters in these galaxies have been studied


"Imaging X-ray Polarimetry Explorer (IXPE) satellite-based mission: instruments, observations, and synergy with other missions"

Grzegorz Madejski (Stanford University and SLAC National Accelerator Center, Stanford, California)

Imaging X-ray Polarimetry Explorer (IXPE) is an international, NASA-led satellite-based mission designed to study polarization of celestial sources in X-ray band, over a bandpass of 2 - 8 keV. The payload, launched in December 2021, consists of three telescopes (built at NASA's Marshall Space Flight Center) focussing X-rays onto polarization-sensitive imaging detectors (built at INFN / Pisa, Italy). This presentation will provide an overview of the mission, and will highlight several early IXPE observations, where the detection (or sensitive non-detection) provides important constraints on the emission processes as well as geometry of celestial X-ray emitters. It will also highlight the synergy of IXPE observations with the data obtained in other wavelengths besides soft X-rays.


"Massive neutron stars observations and theory of dense nuclear matter"

Leszek Zdunik (CAMK, Warsaw)

Recent estimations of the mass of neutron stars indicate that the maximum mass of such objects could be significantly larger (even by about 20%) than the value which appeared to be well established for the last 10 years (two solar masses). I will discuss the possible consequences of such measurements for the properties of dense nuclear matter.


"Accretion disk backflow in resistive MHD simulations"

Ruchi Mishra (CAMK, Warsaw)

Backflow is a part of the flow in the accretion disk which is directed away from the star. Although it is well established in purely hydro-dynamical framework, it is only recently that it has been identified in magnetohydrodynamics (MHD) simulations. We perform resistive magnetohydrodynamics (MHD) simulations of an accretion disk with alpha-viscosity, accreting onto a slowly rotating star endowed with a magnetic dipole. We find backflow in the disk-midplane for a range of conditions when the Prandtl number is less than a critical value. Close to the critical Prandtl number value the backflow shows non-stationary behavior. We compare the results with hydrodynamics simulations. We find that in the MHD case the distance from the star at which backflow begins, the stagnation radius, is different than in the hydrodynamic case.


"On halo and galaxy connection with the cosmic web"

Wojciech Hellwing (Center for Theoretical Physics, PAN, Warsaw)

The Cosmic Web -- an intricate network of clusters, filaments, walls, and voids -- can be identified in a myriad of possible ways leading to a vast ocean of its aspects and properties. We are still quite far-away from possessing widely accepted standards. Because of this the Cosmic-Web impact on the formation and evolution of haloes and galaxies is a subject to ongoing debate. In this talk, I will demonstrate, using the NEXUS+ definition for the Cosmic Web identification, how indeed the disparate environments affect the local formation histories and hence properties of dark matter clumps and galaxies living within. I will focus on internal kinematics and morphological features and distribution of satellite systems. Our findings indicate a fundamental role that the local cosmic environment plays in shaping galaxy and halo formation histories, which in turn have profound implications for their intrinsic properties.

The seminar in a hybrid form. In person in the Lecture Hall at the Copernicus Astronomical Center, and online on Zoom platform and YouTube.


"Heartbeat stars and their tidally excited oscillations among massive stars and red giants"

Piotr Kołaczek (Wrocław University)

The majority of massive main-sequence stars reside in binary or even multiple systems. Moreover, many of them can be found in close and eccentric configurations, i.e. such that they will interact with the companion before the supernova explosion. These facts directly imply that binarity is inherent in the evolution of massive stars and cannot be ignored when studying these objects as well as their final outcomes. For this reason, eccentric ellipsoidal variables (EEVs, aka heartbeat stars) are excellent observational `laboratories' of the ongoing tidal evolution of the aforementioned systems. The orbital phase-dependent tidal potential, acting on both components, can also induce tidally excited oscillations (TEOs) which have quite different properties compared to e.g. self-excited oscillations. Surprisingly, EEVs (and TEOs) can be found even among red giants. These eccentric and evolved systems point to the efficient eccentricity pumping mechanism(s), needing further investigation. During my talk I would like to focus on the latest observational studies concerning EEVs and their TEOs among two distinct groups of stars – massive/intermediate-mass main-sequence stars and red giants. I will mention the `extreme' case of massive EEV, namely MACHO 80.7443.1718, the importance and content of the OGLE collection of heartbeat stars, and the effects of theoretical modeling of TEOs in massive EEVs.


"High-frequency quasi-periodic oscillations and oscillations of accretion disks"

Jiří Horák (Astronomical Institute of the Czech Academy of Sciences)

Accreting black holes and neutron stars show remarkable variability in their X-ray light curves. In this talk, we will concentrate on the high-frequency variability and its possible theoretical explanations. Most of the attention will be devoted to models based on orbital motion and oscillations of accretion disks. In particular, we will discuss basic properties of linear modes of thin disks and their nonlinear excitation.


"Revisiting Asteroseismology with state-of-the-art pre-main sequence models"

Thomas Steindl (University of Innsbruck | Institute for Astro- and Particle Physics)

The pre-main sequence phase of stellar evolution often gets branded as very simple. While the simple classical model of pre-main sequence evolution is incredibly successful, for example, in describing the basic features of young open clusters, the early stellar evolution is much more complicated. The details of the accretion process lead to a chaotic evolution, both in the Hertzsprung Russel diagram and stellar interiors. The advent of space telescopes, in combination with the study of stellar pulsations, provides the opportunity to test stellar structure in an unprecedented way. For asteroseismic studies of young stars, we need to embrace this chaotic phase of stellar evolution in order to harvest its full potential. In this presentation, I will talk about our recent results in the effort to combine asteroseismology and state-of-the-art pre-main sequence models and give an outlook for future research.


"Physical units and the new SI"

Grzegorz Łach (Department of Physics, Warsaw University)

In 2019 the SI system of units underwent a major redefinition — removing any use of physical artifacts as unit standards. Since then all the units are defined based on fundamental physical phenomena. As a consequence several fundamental constants such as the Planck's and the Boltzmann constants have been permanently fixed. I will describe some of the motivations behind this reform, the choices which had to me made, and the use of current definitions in practice.


"Time-domain and Spectroscopic studies of Active Galactic Nuclei"

Saikruba Krishnan (CAMK, Warsaw)

An active galactic nucleus (AGN) is powered by an accreting supermassive black hole (SMBH) at the galaxy's centre and exhibits variability across the electromagnetic spectrum on multiple time scales. Systematic searches for periodic/quasi-periodic signals (QPOs) in AGN have remained an important quest for the astronomy community. I discuss the statistical significance of the detection of periodic/QPOs in red noise-dominated AGN light curves for selected statistical tools (Auto-Correlation Function and the Phase Dispersion Minimisation) and the calibration of their false alarm probabilities. These studies highlight the effects of stochastic variability in AGN light curves and guide the community towards reporting only statistically robust detections. In recent years, apart from the basic characteristic stochastic variations, more extreme variability has been observed, including large outbursts in both X-ray and optical emission. The eROSITA all-sky monitoring mission provides unprecedented monitoring to detect such dramatic changes, and multi-wavelength campaigns help to study the accretion flow response to significant changes in accretion rate and track the corona, disk, and BLR responses. I also present results on such a multi-wavelength study of a flaring event in an AGN detected with eROSITA. We witnessed a likely sudden strong increase in local accretion rate, which manifested itself via an increase in accretion disk emission and thermal Comptonization emission in the soft X-rays, followed by a decrease in accretion and Comptonized luminosity. The physical processes (e.g., disk instabilities) leading to such substantial variations are still an open question, and future continuous monitoring along with multi-wavelength studies will shed some light on it.


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

Arkadiusz Orłowski (Institute of Information Technology, Warsaw University of Life Sciences - SGGW)