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Seminarium środowe


"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)