Wednesday Colloquium



19.01.2022

"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.


26.01.2022

"26-27 January"

CAMK Annual Conference (CAMK, Warsaw)


09.02.2022

"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.


16.02.2022

"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.


23.02.2022

"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.


09.03.2022

"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.


16.03.2022

"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.


23.03.2022

"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.


30.03.2022

"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


06.04.2022

"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.


13.04.2022

"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.


20.04.2022

"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.


27.04.2022

"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


04.05.2022

"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.


11.05.2022

"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.


18.05.2022

"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.


25.05.2022

"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.


01.06.2022

"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.


08.06.2022

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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.


09.06.2022

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Since June 9th - summer break till end of September


05.10.2022

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Marcin Wicha