General Seminar (2026)
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General Seminar takes place every Monday at 11.15 in Piwnice, radioastronomy seminar room.
Remote participation via the BigBlueButton (BBB).
13 April 2026
“Challenges for CMB B modes: Lensing Reconstruction, Subtraction, and Diffuse Foregrounds”
mgr Kishan Deka
The National Centre for Nuclear Research
Abstract:
In the era of high precision Cosmic Microwave Background (CMB) anisotropy measurements, detection of the primordial gravitational waves (PGW) will be one of the main goals of the upcoming CMB experiments. To make it possible, precise cleaning of the CMB polarisation maps from diffused contamination coming from polarised galactic emission and CMB gravitational lensing effect is needed. We investigate the impact of residual galactic foregrounds in lensing reconstruction and delensing of B modes for stage-4 CMB experiments. In this work, we pay special attention to forecast constraints for the amplitude of the primordial gravitational waves and their sensitivity to different foreground models with varying complexity. We are currently exploring a tomographic delensing method on simulations of LSST and Simons Observatory. We aim to achieve optimal delensing efficiency by using a multi-tracer approach, where we split the LSST observations into tomographic redshift bins. We aim to understand the impact of photometric redshift bin mismatch error on the estimation of primordial gravity wave amplitude.
20 April 2026
“Tracing radiation pressure instability signatures in the X-ray light curves and spectra of GRS 1915+105”
dr Kiki Vierdayanti
Bandung Institute of Technology, Indonesia
Abstract:
GRS 1915+105 is widely known as one of the most peculiar Galactic black hole X-ray binaries. It has been observed exhibiting quasi-periodic outbursts with both long and short-term variability in its light curves and spectra for more than twenty years before entering a fainter state in mid-2018. We combined analyses of the short-term light curves with the spectra of GRS 1915+105 using the NuSTAR data (2012–2024) and the long-term MAXI light curves in the 3–20 keV to trace the signature of radiation pressure instability in GRS 1915+105. We discovered a new group of variability classes, which consists of several variability classes, that can be associated with an increased advection rate, which occurs as the mass accretion rate increases even further and the disk becomes geometrically thicker than that of a standard disk but remains optically thick (known as slim disk). Our study showed that highly variable classes that have been studied extensively, such as \rho and \kappa, marked the transition from standard to slim disc, indicating the beginning of the radiation pressure instability. This transition occurs at around 50 per cent of its Eddington luminosity. Meanwhile, the most stable, less variable classes, \chi and \phi2, can be associated with the thermo-viscous instability that occurs in a standard disc, commonly found during the outbursts of the other, less peculiar, black hole X-ray binaries.
27 April 2026
“Modelling tidal decay of hot Jupiters through wave breaking”
mgr Jan Golonka
Institute of Astronomy, Nicolaus Copernicus University
Abstract:
Tidal interactions shape the evolution of close-in giant planets and internal gravity-wave breaking offers an efficient pathway for dynamical-tide dissipation, although its population-wide impact remains poorly constrained. In this talk I will focus on detailed predictions of wave-breaking for a large sample of hot Jupiter systems, discussing both population level trends and results for individual systems. These include predictions for currently operating tidal interactions, and simulations of the orbital evolution of known hot Jupiters until their engulfment or disruption by their host star. Additionally I will present the results of my follow-up observations dedicated to testing the predicted shifts in central times of transits from tidal theory.
4 May 2026
“Cosmic Dipole Anomaly”
prof. Roya Mohayaee
Institut d’Astrophysique de Paris, Sorbonne University, France
Abstract:
I will review the cosmic dipole anomaly, which refers to the discrepancy between the predictions of the FLRW model and observational data. I will explain how this anomaly emerges in analyses of quasars and radio datasets, where it currently exceeds the 5\sigma significance level. I will also review that this result has withstood rigorous systematic tests and persists across multiple datasets, methodologies and analyses.
11 May 2026
“Probing the baryon cycle of primordial galaxies in the ALMA and JWST era”
dr Prasad Sawant
Institute of Astronomy, Nicolaus Copernicus University
Abstract:
The ALMA Large Program ALPINE provides a unique multiwavelength perspective on star-forming galaxies at redshift z ~ 5, offering fundamental insights into their gas and dust content, morphological and kinematic properties, and the physical mechanisms governing their baryonic cycle. In this work, we employ chemical evolution models to investigate the co-evolution of gas and dust within this population of primordial galaxies.
We systematically explore the parameter space of dust production and destruction mechanisms, including Type II supernovae (SNII), asymptotic giant branch (AGB) stars, and grain growth within the interstellar medium (ISM) to reproduce the observed gas and dust properties across the ALPINE sample. Our models successfully account for the gas and dust content of the majority of these galaxies, pointing to SNII as the dominant dust production channel, with galactic outflows and moderate infall of gas playing a key role in regulating the overall dust budget.
A subset of galaxies, however, exhibit an anomalously rapid dust build-up on timescales of order ~ 20 – 100 Myr, which cannot be readily explained within a standard framework. We demonstrate that invoking a top-heavy initial mass function (IMF) which preferentially enhances the formation of massive stars and accelerates dust enrichment partially reconciles this tension, suggesting that variations in the stellar IMF may be a non-negligible factor in the early dust assembly of high-redshift galaxies.
Finally, I will discuss future directions enabled by the latest James Webb Space Telescope (JWST) observations and their synergies with radio continuum surveys through stacking analyses, for constructing a comprehensive picture of baryonic cycle across cosmic time.
18 May 2026
“Propagation of spectrally-resolved cosmic-ray nuclei in the interstellar medium”
mgr Antoine Baldacchino-Jordan
Institute of Astronomy, Nicolaus Copernicus University in Torun
Abstract:
Cosmic rays (CRs) are relativistic charged particles that travel through the interstellar and intergalactic medium. They are a major energetic component of the interstellar medium (ISM) and play a key role in high-energy processes that produce gamma-ray, radio, and X-ray emission. Understanding their propagation is a challenge involving observations, theory, and numerical simulations. Several numerical tools, such as GALPROP, have been developed to study CR physics in galaxies.
A crucial step toward understanding CRs is to model their transport in a magnetohydrodynamical (MHD) environment, which provides a realistic description of galactic dynamics and magnetic field structure. For this purpose, the PIERNIK MHD code and its CRESP (Cosmic Ray Energy Spectrum) algorithm were developed.
In this talk, I will first introduce the basic principles of galactic cosmic-ray physics. I will then present my PhD work on modeling the propagation of spectrally resolved primary nuclei (accelerated in supernovae remnants) and secondary nuclei (produced by collision of primaries with the ISM atoms) in a dynamical MHD framework using PIERNIK and CRESP. I will show results for secondary-to-primary ratios and unstable-to-stable isotope flux ratios, which can be used to constrain CR transport parameters. These results demonstrate that the observables depend sensitively on the coupling between CRs and the ISM. I will also present new results on CR propagation in resistive magnetic fields, highlighting that local magnetic turbulence in the ISM may strongly influence CR transport.
25 May 2026
“Do LLMs Reason?”
prof. Ulrike Hahn
Birkbeck Univ London
Abstract:
Few debates have been as polarised as the debate about “reasoning” in LLMs. The extent of the disagreement seems noteworthy in and of itself and in need of explanation. The talk details recent work (Hahn, 2026) seeking to clarify underlying terminological and conceptual disagreements that give rise to these seemingly irreconcilable positions.
Preprint: Hahn, 2026, https://zenodo.org/records/18231172
1 June 2026
“Position of X-ray low surface brightness clusters in the cosmic filament network: dependence on filament catalog selection”
dr hab. Mariusz Tarnopolski, prof. UMK
Institute of Astronomy, Nicolaus Copernicus University
Abstract:
Zarattini et al. [Astronomy and Astrophysics, 694 (2025), A256] recently reported that core-excised X-ray surface brightness, SB_X, of low surface brightness clusters (LSBCs) exhibit an anti-correlation with their distance to nearest galactic filament, D_fila. We determine the position of LSBCs with respect to the cosmic network by employing three different filament catalogs. Utilizing the X-ray Unbiased Cluster Sample (XUCS), we investigated how D_fila depends on SB_X for the three filament catalogs. The XUCS samples were relatively small, ranging from 17 to 29 LSBCs. We took into account the uncertainties in SB_X and performed Monte Carlo simulations to assess the probability of a particular pattern being a chance occurrence. We obtain a positive correlation, anti-correlation, and no correlation depending on the filament catalog utilized. Using a permutation test we estimate that, given the sample sizes, there is a 40%-60% probability that the observed patterns are a chance occurrence. By varying the measured values within their standard errors, we find that there is a further 6%-8% probability that the patterns present in the data are spurious. The relation between D_fila and SB_X depends on the filament catalog used: there can be a positive correlation, anti-correlation, or no correlation. This sensitivity indicates that systematic differences in filament finding algorithms can significantly affect environmental metrics such as D_fila. All in all, the current samples are too small to infer the character of the relation between SB_X and D_fila.
8 June 2026
“On the similarity of cosmic structures”
prof. Matthias Bartelmann
Institut für Theoretische Physik, Universität Heidelberg
Abstract:
The Universe is filled with structures of vastly different scales: galaxies themselves span many orders of magnitude in mass and size; they agglomerate in galaxy groups and clusters; and they align along filaments tens of millions of lightyears long. Notwithstanding their vastly different sizes, these gravitationally bound structures are internally very similar: for example, the radial profiles of their dominant, dark-matter density differ by scale, but not by shape. Why is this so, and what can we learn from this kind of universality? Does it depend on the nature of dark matter, details of the cosmological model, or of gravity theory? The talk will discuss approaches to these questions, present partial answers, and lead to conjectures about cosmic structures that remain to be tested.