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Seminarium środowe


"Magnetic flux saturation mechanism at accreting black holes"

Krzysztof Nalewajko (CAMK, Warsaw)

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.


"Population of exoplanets found using microlensing technique"

Radek Poleski (Astronomical Observatory Warsaw University)

There are four main methods of finding exoplanets: transits, radial velocity, microlensing, and direct imaging. Each of these methods comes with its own biases and limitations. The microlensing technique allows finding planets on orbits similar to the ones in the Solar System as well as free-floating planets (i.e., not bound to any star) of low mass. I'll present how the microlensing method works and what are the current constraints on the population of bound and free-floating planets. I'll also present prospects for a microlensing survey that will use the Nancy Grace Roman Space Telescope - a NASA flagship mission currently built and scheduled to be launched in 3 years.


"The impact of magnetic field on beamed emission of neutron star ULXs in Radiative GRMHD simulations"

Fatemeh Kayanikhoo (CAMK, PAN, Warsaw)

In our simulations accreting magnetized neutron stars are considered to be potential pulsating ultraluminous X-ray sources. Due to the neutron star's magnetic field, the emission is beamed, resulting in apparent luminosity that exceeds the Eddington luminosity expected from stellar mass objects. In my research, I investigate the impact of the surface magnetic field of a neutron star on the photosphere and luminosity by using GRRMHD simulations. Our study shows that strong magnetic fields truncate the disk and quench the outflows, which lowers the luminosity. Moreover, our simulations demonstrate that the beaming emission decreases as the magnetic field increases.


"A unified accretion ejection paradigm for X-ray binaries: the JED-SAD paradigm"

Grégoire Marcel (CAMK, Warsaw)

The hysteresis behavior of X-ray binaries during their outbursts remains a mystery. In this work, we developed a paradigm where the disk material accretes in two possible, mutually exclusive, ways (Ferreira et al. 2006). In the usual alpha-disk mode (SAD, Shakura & Sunayev 1973), the dominant local torque is due to a radial transport of the disk angular momentum. In the jet-emitting disk mode (JED), magnetically-driven jets carry away mass, energy, and angular momentum vertically. Within this framework, the transition from one mode to another is related to the magnetic field distribution, an unknown. We have shown that typical hard states of X-ray binaries can be reproduced up to unprecedented X-ray luminosities in this paradigm (Marcel et al. 2018a,b,2019). Direct spectral fits have since been performed on an AGN (Ursini et al,. 2020), as well as two X-ray binaries (Marino et al. 2021, Barnier et al. 2022), showing striking dynamical similarities between the two accretion flow structures despite the factor > 10^6 in mass. Moreover, we have addressed the production of low frequency quasi-periodic oscillations during the outbursts (Marcel et al. 2020, Marcel & Neilsen 2021), the radiative efficiency of the accretion flow and the associated radio--X-ray correlation (Marcel et al. 2022), as well as the production of winds (Petrucci et al. 2021), the latest results from IXPE (Zhang et al., in prep.), and the timing properties of X-ray binaries (Malzac & Marcel, to be submitted).