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Seminarium odbywa się w Piwnicach w sali wykładowej budynku Radioastronomii. Początek o godzinie 11.15.
Udział w seminarium w trybie zdalnym za pośrednictwem BigBlueButton (BBB).
26 lutego 2024
“Tangle of Chaos in Standing Gravitational Waves”
mgr Syed Naqvi (Obserwatorium Astronomiczne, Uniwersytet Jagielloński)
Standing waves, well-studied in mechanical and electromagnetic domains, have been studied within the Einstein-Rosen spacetime, representing cylindrical standing gravitational waves. Our study aimed to analyze the behavior of test particles’ orbits within this spacetime. We observed a sensitivity to initial conditions in the trajectories of these particles. Additionally, we discover the fractal associated with this system. Examining the Poincare map, we identified intricate crossings of stable and unstable manifolds at hyperbolic fixed points. A chaotic heteroclinic network further characterized the complex dynamics of massive test particles. Analogous to phenomena such as mechanical vibrations generating Chladni figures and the intricate shapes of Faraday waves, gravitational standing waves produce complex patterns through the interactions of test particles.
4 marca 2024
“Clouds vs Cherenkov telescopes: problems and solutions”
prof. dr hab. Julian Sitarek (Pracownia Symulacji Komputerowych, Uniwersytet Łódzki)
Very-high-energy (VHE >~100 GeV) gamma rays are absorbed in Earth’s atmosphere and thus cannot be detected directly on Earth. Their fluxes are also typically too low to efficiently study them with satellite instruments. A VHE gamma ray entering the atmosphere initiates an electromagnetic cascade that induces faint flashes of blueish Cherenkov light. Such flashes can be then detected by Imaging Atmospheric Cherenkov Telescopes registering images of passage of individual gamma rays through the atmosphere. The usage of atmosphere as a part of the detector allows us to achieve a collection area of gamma rays over two orders of magnitude higher than the physical size of the detector. But it also introduces systematic errors connected with the atmosphere’s transparency. In particular, cloud presence during the observations can significantly affect the data. In this seminar I will cover different methods used to correct the influence of the clouds. I will show how lack of such a correction introduces bias in the energy estimation of gamma rays. I will present how the affected images of showers are degraded and thus can be confused with background events, lowering the collection area of the telescope. Finally, I will show a novel method of correcting the influence of the clouds already at the image level, and discuss the possibility of measuring the parameters of a cloud directly with the observations by the Cherenkov telescopes.
11 marca 2024
“4MOST: the 4-metre Multi-Object Spectroscopic Telescope”
prof. dr hab. Boudewijn Roukema (Instytut Astronomii, Uniwersytet Mikołaja Kopernika)
18 marca 2024
“Magnetic flux saturation mechanism at accreting black holes”
dr hab. Krzysztof Nalewajko, prof. CAMK PAN (CAMK PAN)
Black holes (BH) acquire relativistic magnetospheres by accreting magnetized gas. Once they collect significant magnetic flux across the horizon, aided by the spin they can drive powerful relativistic jets by the Blandford-Znajek mechanism. Large enough BH magnetic flux backreacts on the accretion flow, which has been described in terms of arresting or choking. Magnetic flux eruptions have been identified as the mechanism of BH magnetic flux saturation. These eruptions can potentially dissipate a large fraction of magnetic energy in the BH magnetosphere by means of relativistic magnetic reconnection, accelerating particles and producing flares of non-thermal radiation. We analyze the results of 3D general-relativistic ideal magnetohydrodynamic (GRMHD) numerical simulations of accretion flows onto magnetically saturated Kerr BHs, focusing on the initiation of magnetic flux eruptions.