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Seminarium środowe



24.04.2024

"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).


08.05.2024

"On the hot flows in AWDs: broadband noise, spectral characteristics and disk structure"

Solen Balman (Department of Astronomy and Space Sciences, Istanbul University)

Cataclysmic Variables (CVs) and related systems (e.g., AM CVn, Symbiotics) are compact systems with white dwarf (WD) primaries referred as accreting white dwarfs (AWDs). They are excellent laboratories to study astrophysical plasmas, accretion flows and disks, gas dynamics, outflows, and transient outbursts. Broadband noise and its variations in accretion flows have been a diagnostic tool for understanding the structure of accretion disks together with accretion history and state changes. CVs demonstrate band limited noise (mainly 1-6 mHz) in the optical, UV and X-ray energy bands, which can be adequately explained in the framework of the model of propagating fluctuations yielding break frequencies. I will discuss broadband noise structure in CVs with a broader sense including some different nonmagnetic AWDs elaborating on similarities along with spectral characteristics. The spectral and/or broadband noise studies show that advective hot flow structure (ADAF-like) resides inside nonmagnetic CV disks mainly detected in the X-ray regime (Balman 2020, Balman et al. 2022 and references therein) indicating other characteristics like outflows in the X-rays and warm absorber effects.


15.05.2024

"Using the Alcock-Paczyński test to constrain the amplitude of the cosmological constant"

Andrzej Sołtan (CAMK, Warsaw)

The galaxy autocorrelation function (ACF) is constructed using the galaxies selected from the DR12 of the SDSS. The ACF amplitude increase at separa- tions expected to those generated by the Baryon Acoustic Oscillations (BAO) is clearly detected. We study the dependence of this correlation signal on the angle between the line of sight and vectors defined by galaxy pairs. Using the Alcock-Paczyński test we estimate acceptable amplitudes of the matter and cosmological constant density parameters, Ωm and ΩΛ. Only flat cosmological models are considered, e.g. Ωm + ΩΛ = 1. We found that in the local Universe 0.25 . Ωm . 0.45, what is in agreement with the results based the CMB me- asurements. Strong stoochastic variations of local galaxy concentrations generate substantial scatter of the ACF signal unrelated to the BAO, what significantly degrades the accuracy of the present estimates.