Wednesday Colloquium


"Cepheids in the far side of the Milky Way from the VVV survey"

Gergely Hajdu (NCAC, Warsaw)

The structure of the far side of the Milky Way's disk has not been charted until very recently, due to the severe extinction caused by interstellar dust in the Galactic plane. In order to do so, we have conducted a census of distant Classical and Type II Cepheid variables along the Galactic plane in the VISTA Variables in the Vía Láctea (VVV) survey. Zero-point calibration issues of VVV survey products forced us to revise them, after which the photometry could finally be leveraged to its fullest potential. We have searched the recalibrated VVV Ks-band time series of the Galactic plane area for Cepheid variables, which have been classified using a convolutional neural network, revealing 640 Classical and over 500 Type II Cepheids. The Type II Cepheids were used to probe the near-IR extinction law, as well as the structure of the ancient Galactic inner bulge. Similarly, the Classical Cepheids are used to trace the distribution of the young stellar component of the far Galactic disk.





"How electron-positron pair plasma fills pulsar magnetosphere, heats NS surface, and generates radio emission."

Andrey Timokhin (Uniwersytet Zielonogórski)

I give an overview of recent pulsar magnetosphere models emphasizing the importance of pair plasma generation. I discuss how much pair plasma can be produced in pair cascades and what it means for the physics of Pulsar Wind Nebulae. Relativistic particles accelerated in pair formation zones heat the neutron star surface, I demonstrate that the temperatures of pulsar polar caps predicted in the frame of modern pair cascade models agree with observations quite well. I also present a novel robust mechanism for the direct generation of coherent radio emission in pair discharges and discuss its implications.


"Twenty Years of Science with the Chandra X-ray Observatory"

Aneta Siemiginowska (CfA, Harvard University)

X-ray emission traces signatures of energetic and violent events associated with formation and evolution of all structures in the universe. Studies of X-ray sky are relatively recent, as the first X-ray cosmic sources were detected only sixty years ago, while the optical sky has been known to humans since the beginning of life. Over the past two decades, our view of the X-ray universe has been significantly improved thanks to the NASA’s Chandra X-ray Observatory. Chandra, with its extremely sharp X-ray vision enabled by large-area, sub-arcsecond mirrors, has revolutionized our understanding of the high-energy sky. It has brought many important astrophysical processes into focus for the first time and has comprised a key component of our multiwavelength view of the Universe. In this talk I will give a broad overview of the science enabled with Chandra and focus on a handful of the exciting results, including breakthrough discoveries on the births and deaths of stars, the cosmic growth of black holes, and the formation and evolution of galaxies and clusters. I will conclude with a look toward the future of high-resolution X-ray observatories.


"Quantum communication in space"

Piotr Kolenderski (Uniwersytet Mikołaja Kopernika, Toruń)

Quantum communication offers a selection of methods for absolutely secure exchange of information. There are two particular links which are used in practice: fibers and free space. The latter implemented using satellites is more challenging, but offers substantially longer ranges. During my talk I will present two projects running in our lab at Nicolaus Copernicus University, which are related to satellite based quantum communication. The first one aims in building a ground station for a satellite receiver link. The second one is a joint effort with Syderal Polska, where the goal is to build a satellite-grade polarization entanglement controller.


"Echoes from the sky: X-ray reverberation around BHs"

Barbara de Marco (CAMK, Warsaw)

What is the geometry of the accretion flow around accreting black holes (BH), and how does it evolve with the accretion state of the source? Time delays associated with the reprocessed signal from gas orbiting the BH allow us to map the innermost regions of accreting-BH systems. This technique, called X-ray reverberation, is shedding some light on these outstanding questions. I will be discussing the state-of-the-art in this field.


"Comparison of LIGO/Virgo data with stellar and binary evolution models"

Krzysztof Belczyński (NCAC, Warsaw)

I will discuss the astrophysical importance of the recent LIGO/Virgo direct detections of BH-BH, BH-NS and NS-NS in gravitational-waves. For 10 BH-BH merger detections new models of isolated classical binary evolution can recover LIGO/Virgo merger rates, BH masses and spins. This still does not exclude other formation channels, and we do not know yet how these BH-BH mergers have formed. However, the recent detection of NS-NS merger in an old elliptical host galaxy can not be reproduced by any major formation channel. Despite the fact that the exact origin of LIGO/Virgo sources is not yet known, several astrophysical implications are beginning to emerge.


"Modeling 2009-2013 observations of M87* with an asymmetric ring"

Maciej Wielgus (Harvard Smithonian CfA)

The Event Horizon Telescope (EHT) has delivered first resolved images of M87*, a supermassive black hole in a center of the M87 galaxy. These results are based on the 230 GHz observations performed in April 2017. Dedicated tools were developed to facilitate the modeling and analysis of the EHT 2017 data set in a Markov chain Monte Carlo (MCMC) framework, demonstrating that the source morphology can be very well represented by a crescent. More data, from different time epochs, are required to investigate the long term stability of the source parameters, such as its diameter and orientation. To address that need, we analyze archival data from proto-EHT 230 GHz observations of M87* in the framework utilized for the 2017 data analysis. We fit geometric models to the observations taken in 2009, 2012 (published in the past) and 2011, 2013 (not published previously), exploring the parameter space with a MCMC algorithm. We validate the procedure using synthetic data. While the archival data sets are far less constraining than the 2017 observations, we obtain measurements of the source diameter and orientation. Variations of the M87* morphology in the 2009--2017 period are found to be roughly consistent with the predictions of general relativistic magnetohydrodynamic simulations of a turbulent accretion flow.


"Science with Cherenkov Telescope Array (Colloqium on-line only, see attached info)"

Julian Sitarek (Uniwersytet Łódzki)

Colloquium on-line. Join Zoom Meeting For details on the connection mail to General information: During the Colloquium, please keep your cameras off and microphones mute (except for asking the question).

Colloquium on-line.  

Join Zoom Meeting      
For details on the connection mail to Stanisław Bajtlik.  

General information:

During the Colloquium, please keep your cameras off and microphones mute (except for asking the question).


"Accretion onto black hole modeled via relativistic MHD simulations (Colloquium on-line only)"

Agnieszka Janiuk (CFT PAN)

I will present the recent works of the relativistic astrophysics group in CTP PAS, related to modeling of cosmic sources containing accreting black holes. Numerical methods developed recently in the framework of the code HARM-COOL, have applications to the studies of gamma ray bursts, collapsars, and kilonovae.

Colloquium on-line.
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"Pulsing and Non-Pulsing ULXs: the Iceberg Emerges"

Jean-Pierre Lasota (IAP, Paris, CAMK, Warsaw)

Ultraluminous X–ray sources (ULXs) are objects with apparent (i.e assumed isotropic) luminosities Lapp > 1039 erg/s, exceeding the usual Eddington value for stellar– mass black holes, but which do not contain supermassive black holes. I will show that ultraluminous X-ray sources (ULXs) with coherent X-ray pulsing (PULXs) have normal (X-ray pulsar strength) magnetic fields and therefore their apparently super-Eddington luminosity must be geometrically beamed. I will present the model describing the physical properties od PULXs and show that they probably have neutron-star spin axes significantly misaligned from their central accretion discs. Scattering in the funnels collimating their emission and producing their apparent super-Eddington luminosities is the most likely origin of the observed correlation between pulse fraction and X-ray photon energy. Pulsing is suppressed in systems with the neutron-star spin closely aligned to the inner disc, explaining why some ULXs show cyclotron features indicating strong magnetic fields, but do not pulse. I will argue that alignment (or conceivably, field suppression through accretion) generally occurs within a fairly short fraction of the ULX lifetime, so that most neutron-star ULXs become unpulsed. This further suggest that almost all ULXs actually have neutron-star accretors, rather than black holes or white dwarfs, reflecting their progenitor high-mass X-ray binary and supersoft X-ray source populations.

Colloquium on-line.
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"Chemical compositions as indicators of evolution in binary systems. Results from studies of symbiotic giants"

Cezary Gałan (CAMK, Warsaw)

Symbiotic stars are a type of evolved, long-period binary systems composed of a cool, giant donor and a hot, compact object - usually a white dwarf. The complex nature of interactions between the components of these systems makes them ideal for studying evolution in binary systems, especially at the final stages. Chemical composition, next to initial mass is among the major parameters determining stellar evolution. Until recently, knowledge of abundances in symbiotic red giants was limited to only a few cases. Our analyzes of the last several years, using high and low-resolution spectra in the optical and especially infrared domain not influenced significantly by the nebular continuum, increase statistics to well over one hundred objects. Our spectroscopic data and methods used will be presented. I will discuss what we can learn about the evolution of components of the long-period binary systems, the history of the mass transfer and evolution in Galactic stellar populations on the example of studies of chemical composition in symbiotic giants.

Colloquium on-line.
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"What can we infer from black hole images?"

Frederic H. Vincent (Observatoire de Paris)

The recent Event Horizon Telescope (EHT) millimeter image of the vicinity of the supermassive compact object M87* at the center of the galaxy M87 is a breakthrough. It is important to investigate what can and what cannot be inferred from such images. In this talk I will discuss a simple geometric model of the region of the accretion flow immediately surrounding M87*. I will use this model to discuss the highly-lensed features of EHT-like images, often loosely referred to as the "photon ring". I will discuss whether such features can be used as clean probes of the nature of the underlying spacetime. I will also discuss various EHT-like images of M87*, assuming this object is various kinds of compact alternatives to the standard black hole. This will allow discussing to what extent can EHT-like images help constrain the nature of the compact objects at the center of M87 or our Milky Way.

Colloquium on-line.
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"Puffy accretion disks and how to find them"

Maciej Wielgus (Center for Astrophysics | Harvard & Smithsonian)

A widely accepted picture of an accretion flow in a soft spectral state X-ray binary is a geometrically thin disk structure much alike the classic analytic solution of Shakura & Sunyaev. Despite the fact that the analytic models are troubled by instabilities and miss important aspects of physics such as magnetic fields, they are commonly and successfully used as a framework for interpreting the observational data through spectral fitting. The thin disk model applicability to observational data analysis breaks for luminosities > 0.3 Eddington luminosity, when disk becomes thicker and advection becomes appreciable. In recent years global relativistic radiative magnetohydrodynamic simulations have reached sufficient maturity to probe the regime of optically thick accretion with a mildly sub-Eddington luminosity. These simulations find the accretion flow stabilized by the magnetic field, with a dense geometrically thin core and a puffed-up optically thick region around it, reaching h/r of ~1 for half of the Eddington luminosity. Interactions between the magnetic field and radiation create a non-trivial vertical structure of the flow, that is not captured by the (semi)analytic models. I will discuss these recent simulations and their potential observational implications, including the collimation effects, and influence on the spectra.

People from outside of the Copernicus Center are very welcome to participate. For technical detailes please contact Stanislaw Bajtlik at:  


"Accretion disc inner radii in black-hole binaries"

Andrzej Zdziarski (CAMK, Warsaw)

I will present a number of results related to the relative importance of relativistic effects in accreting flows onto black holes. In particular, I will review the current evidence for and against the accretion disc extending to the innermost stable circular orbit (ISCO) in the hard spectral states of black-hole binaries. The spectral and timing results of our group imply the discs are significantly truncated away from the ISCO in the hard state. Especially, a recent result of Zdziarski & De Marco (arXiv:2002.04652) rules out the presence of a disc near the ISCO in luminous hard states based on the Stefan-Boltzmann law. On the other hand, the majority of currently published papers claim the disc is extending to the ISCO and very strongly irradiated.

People from outside of the Copernicus Center are very welcome to participate. For technical detailes please contact Stanislaw Bajtlik at:  


"Resonant structure of galaxy discs"

John Beckman (Instituto de Astrofísica de Canarias)

In our research group we have built and used a Fabry-Perot interferometer which, on the 4.2m William Herschel Telescope at the IAC Observatory on La Palma, gives two-dimensional velocity maps of galaxies using the Halpha emission from the interstellar medium. The maps have a wide field (3.4 x3.4 arcminutes), with seeing-limited spatial resolution below 1 arcsecond and velocity resolution of order 6 km/s across the full field. With these maps we have devised an accurate method for determining the corotation radius of a density wave pattern in a galactic disc. Applying the method we find that the discs have multiple corotation radii: the density waves are confined in separate annular regions, not necessarily related to the galactic bar. These results are being applied to a number of problems: whether galactic bars are being slowed down by dynamical friction with dark matter halos; whether swing amplification or manifold scenarios give a better account of the generation of the spiral arms; what determines the numbers and the bifurcations in the arms. My talk will give an overview of this subject, and present the latest results.

People from outside of the Copernicus Center are very welcome to participate. For technical detailes please contact Stanislaw Bajtlik at:  


"Stellar-merger remnants: how stars collide and what is the aftermath"

Tomasz Kamiński (CAMK, Toruń)

People from outside of the Copernicus Center are very welcome to participate. For technical detailes please contact Stanislaw Bajtlik at:


"The Hubble Tension"

Krzysztof Bolejko (University of Tasmania)

One of the most important cosmological discoveries of the last century was the discovery of the expansion of the universe. The expansion rate is measured in terms of the Hubble constant. Throughout the 20th century measurements of the Hubble constant improved. With the improved precision it transpired that high redshift constraints on the Hubble constant are in tension with local measurements. In the recent years this tension has become so prominent that it is now often argued that it will lead to a change of the standard cosmological model (just like in 1990s when supernovae date initiated a shift from the Einstein-de Sitter model to the Lambda-CDM model). In my talk I will discuss the tension in measurements of the Hubble constant. I will also talk about possible directions and avenues of solving this tension.


"Heritage in troubling times"

Monika Stobiecka (Artes Liberales, Warsaw University)

Contemporary age poses serious threats to various material heritage objects and monuments. The climate change, growing nationalism, military conflicts, as well as quick technological progress, redefine our realities and affect the ways in which we perceive heritage. No longer can we think about traditional forms of protected monuments, when those are exposed to instrumentalization, abrupt changes and/or destruction. In my presentation, I would like to discuss three important factors shaping today’s heritage: politics, technologies and climate change. Those problems will be illustrated with my previous researches on art and archaeology, namely the Zakopane style (politics), the copy of the Syrian Arch (technologies), and the ruins of Petra (climate change).


"Wind fed accretion onto Cygnus X-1"

Ishika Palit (Center for Theoretical Physics, PAN, Warsaw)

Cygnus X-1 is a black hole X-ray binary system with a companion, supergiant star, HDE-226868. It is one of the brightest X-ray sources observed and shows the X-ray intensity variations in both the soft and hard X-rays. The focused stellar wind from the supergiant provides the source of the matter accreting into the black hole and thus also affects the observed spectral states. I will present my recent work on 2D numerical modeling replicating such focused, clumpy wind from the binary companion fed for accretion onto the black hole. Further, I will discuss the role of shock oscillation in the aspect of the observed variability in Hard X-rays and will present our results from Power Density Spectral (PDS) analysis


"Towards Understanding Black Hole Accretion and Jet Launching"

Monika Mościbrodzka (Radboud University, Nijmegen)

The Event Horizon Telescope (EHT) is a global effort to construct an Earth-sized virtual radio telescope array, with the ultimate goal to actually make pictures and movies of some nearby supermassive black holes. The initial results of the first full EHT observing run in 2017 were presented on 10 April 2019. A detailed theoretical understanding of black hole astrophysics is now very crucial to interpret these observations. The focus of the talk is on modeling intensity and polarimetric properties of light produced in synchrotron processes in plasma falling towards the event horizon. In particular the polarized component of light gives us detail constraints on the magnetic field geometry and dynamics at the event horizon, which are keys to understand the accretion and jet launching process.


"The Size, Shape, and Scattering of the Black Hole Sagittarius A*"

Sara Issaoun, (Radboud University, Nijmegen)

The Galactic Center supermassive black hole, Sagittarius A* (Sgr A*), is the most promising target to study the dynamics of black hole accretion and outflow via direct imaging. In April 2017, the Atacama Large Millimeter/submillimeter Array (ALMA) operated as a phased array in its first science run with very long baseline interferometry (VLBI). As part of the extensive multi-wavelength campaign for the Event Horizon Telescope, we observed Sgr A* at 3.5-mm with the Global Millimeter VLBI Array (GMVA) and ALMA, reaching an angular resolution of 87 micro-arcseconds, double that of previous experiments at the same wavelength. In this talk, I will present the observations that enabled the first imaging of the intrinsic structure in Sgr A* at 3.5-mm, revealing a nearly isotropic source. I will also discuss the constraints on source size, shape and interstellar scattering that these observations have provided and how these inform horizon-scale expectations for the Event Horizon Telescope.


"Manifestations of MADness (in Accretion Disks)"

Mitch Begelman (JILA, University of Colorado, Boulder, CO)

Magnetically arrested accretion disks (MAD) have been implicated in the formation of powerful relativistic jets by accreting black holes. The basic idea is that the ram pressure of the accretion flow confines a maximum amount of magnetic flux around the black hole, optimizing the extraction of black-hole spin energy. But the situation outside the black hole may be more complicated, with the toroidal magnetic field - not the poloidal flux - dominating the transition to MADness and the resulting disk structure. I will discuss various factors that may come into play when an disk goes MAD, and how recent observations can help with the diagnosis.

Due to the time difference, the lecture will be held at 5:15 pm Warsaw time.


"Blazar Flare and Theoretical Modeling of Multiwavelength Emission"

Raj Prince (Center for Theoretical Physics, PAN, Warsaw)

Blazars are a class of an AGN with jet pointing towards the observer within a few degrees, and due to relativistic effects, all the emission is boosted along the jet axis. They show the spectacular flares across all the wavebands ranging from radio to very high energy gamma-ray. They also offer very strong variability on the time scale of minutes to years during the flaring activity. The reason behind the fast variability and high flaring states are still unknown. However, the broadband study can answer these questions to some extent. The broadband SED of blazar shows a two hump kind of structure one peaks in low energy band and the other in high energy band. I will discuss a few proposed models to explain the fast variability, and further, will talk about the possible physical mechanisms responsible for the multi-wavelength emission.


"The Planck cosmological legacy"

François R. Bouchet (Institut d'Astrophysique de Paris)


"Mono-enriched stars and Galactic chemical evolution -- Possible biases in observations and theory"

Camilla J. Hansen (Max Planck Institute for Astronomy, Heidelberg, Germany)

A long sought for goal using chemical abundances is to understand the chemical evolution of the Galaxy and to pin down the nature of the first stars (Pop III). Metal-poor, old, unevolved stars are excellent tracers as they preserve the abundance pattern of the gas they were born from. Here we use a sample of 14 metal-poor stars observed at high resolution with PEPSI/LBT, to derive abundances of 32 elements. In this talk I present well-sampled abundance patterns for all stars obtained under the assumption of 1D, local thermodynamic equilibrium (LTE). To infer the nature of these stars, we compare unevolved cool stars, which have been enriched by a single event (``mono-enriched''), with yield predictions to pin down the mass and energy of the Pop III progenitor. A simple fit may bias our inferred mass and energy just as much as the simple 1D LTE abundance pattern. We therefore pursue with an improved fitting technique considering dilution and mixing and non-LTE corrected abundances. To date only few mono-enriched stars are known. Our sample presents Carbon Enhanced Metal-Poor (CEMP) stars some of which are promising bona fide second generation (mono-enriched) stars. Finally, we explore the predominant donor and formation site of the rapid and slow neutron-capture elements.


"Modifying the Inverse-square law of radiation for close-in exoplanets"

Mradumay Sadh (CAMK, Warsaw)

The inverse square law for calculating irradiance fails to predict the irradiation for planets close to their host stars. This makes it challenging to model the climate of these exoplanets through present GCM models. This talk would be about the attempt to revise the assumptions of this law and estimate correct values of irradiance for such planets with a geometrical analysis. Reference paper -


"How important is bulk viscosity in neutron star mergers?"

Mark Alford (Washington University in Saint Louis)

Neutron star mergers are laboratories for hot and dense nuclear matter. In a merger, the stellar material experiences large changes in temperature and density that happen in milliseconds. This means that mergers probe dynamical properties that may help us uncover the phase structure of ultra-dense matter. I will describe some of these properties, and then focus on the most promising, bulk viscosity. I will explain how bulk viscosity arises and how it might have a significant effect in a merger.

Due to the time difference the talk will be at 4:15 pm.


"Putting Infinity on the Grid"

David Hilditch (CENTRA, Technical University of Lisbon)

I will talk about an ongoing research program relying on a dual frame approach to treat numerically the field equations of GR (in generalized harmonic gauge) on compactified hyperboloidal slices. These slices terminate at future-null infinity, and the hope is to eventually extract gravitational waves from simulations there. The main obstacle to their use is the presence of ’infinities’ coming from the compactified coordinates, which have to somehow interact well with the assumption of asymptotic flatness so that we may arrive at regular equations for regular unknowns. I will present a new ’subtract the logs’ regularization strategy for a toy nonlinear wave equation that achieves this goal, and outline how this strategy can be applied to GR.


"Spectroscopic characterisation of very young systems in our Galaxy: from stellar clusters and young associations, down to sub-stellar objects."

Valentina D'Orazi (INAF - Astronomical Observatory of Padova, Italy)

Large spectroscopic surveys are (and will be ) providing us with a plethora of information on atmospheric parameters and detailed chemical composition (light, proton-capture, iron-peak and neutron-capture elements) in different populations across the Galactic components (bulge, thick disk and thin disc) for field and cluster stars. Interestingly, less attention has been devoted to the study of the chemical composition of very young stars (hereinafter stars younger than ~100 - 200 Myr). Two major issues are currently affecting our comprehension of these systems: the lack of young metal-rich stars/clusters in the solar surrounding and the so-called barium puzzle. I will review the status of the field and will present new results on both topics. General implications on Galactic chemical evolution and planet formation will be discussed. Finally, I will present SHARKs -a new facility with coronagraphic and spectroscopic capabilities that is under construction at INAF and will be mounted at LBT (Arizona, US) during Summer 2021. Scientific cases include exoplanet detection and characterisation, circumstellar discs and jets, AGN and solar system bodies.


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

Marek Abramowicz (CAMK, Warsaw)