ABSTRACTS
Author: Michal Bursa (Astronomical Institute, Czech Academy of Sciences, Czech Republic)
Title: Effects of geometry and mass accretion rate on thermal spectra of ULX sources
Spectra of many ULX sources show a characteristic shape of double-peaked thermal spectra, where the first peak is most likely produced by true thermal contribution from the disk while the second peak probably comes from the thermal comptonization. The talk is going to show how the the observed shape of the thermal component is affected by geometrical setup of the system also in relation to the mass accretion rate and how the thermal spectrum changes as a result of thermal comptonization.
Author: Caballero-Garcia M. D. (Astronomical Institute, Czech Academy of Sciences, Czech Republic)
Co-authors: Michal Dovčiak
Title: The very first results from the use of the X-ray reverberation model KYNREFREV in XSPEC
X-ray reverberation mapping has been revealed to be a valuable tool for knowing the physical condition of the accreting black holes and the matter that surrounds them. This is an important case of interest for the exploitation of the data from the next generation of big X-ray satellites (Athena). A new model has been developed by our group for the use with X-ray astronomical data, mainly through both timing and spectroscopy techniques. Here we present this new XSPEC model, KYNREFREV, and some preliminary results obtained by using it.
Author: Michal Dovčiak (Astronomical Institute, Czech Academy of Sciences, Czech Republic)
Co-authors: Marin F., Goosmann R. W., Svoboda J., Di Lalla N., Karas V., Matt G.
Title: Influence of a polarized primary source on the X-ray polarization resulting from disc reflection in AGN
Theoretical computations showed that the reflection of X-ray radiation from the accretion disc in AGN should result in significant (detectable) polarization signals. Originating from a primary power-law coronal emission situated above the disc surface, X-ray photons are partially reprocessed by Compton scattering in the disc material and show a polarization level that heavily depends on geometry of scattering. In this contribution, we examine the polarization that can be obtained in the lamp-post geometry scenario, where a compact patch of corona is positioned on the axis above the black hole. Up to now only unpolarized primary emission was considered. We will show the results of a more likely scenario where the primary X-ray photons are already partially polarized by up-scattering in the hot corona. The influence of differently polarized primary source will be presented and simulated observations with the X-ray polarimetry mission XIPE, proposed as M4 ESA mission, will be shown.
Author: Kateřina Goluchová (Research Centre for Computational Physics and Data Analysis, Institute of Physics, Silesian University in Opava, Czech Republic)
Co-authors: Gabriela Urbancová, Gabriel Török, Martin Urbanec
Title: ISCO frequencies for rotating neutron stars - simple and accurate formulae
Keplerian frequency at the innermost stable circular orbit (ISCO frequency) has been within several astrophysical studies frequently assumed as the characteristic orbital frequency. For simplicity, the relativistic formula linear in neutron star angular momentum is often considered when expressing its value for a rotating neutron star. The ISCO frequency however depends on the interplay between strong gravity effects and effects given by neutron star's oblateness. Consequently, for a given neutron star mass and equation of state this frequency may increase as well as decrease when the spin and angular momentum of the star increase. We present simple fitting quadratic formulae for the ISCO frequency. We further present formulae based on universal neutron star relations.
Author: Jiří Kovář (Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava, Czech Republic)
Title: Astrophysical levitation
Author: Debora Lančová (Research Centre for Computational Physics and Data Analysis, Institute of Physics, Silesian University in Opava, Czech Republic)
Co-authors: Pavel Bakala, Mauricio Falanga, Luigi Stella, Alessandra de Rosa, Vittorio de Falco, Kateřina Goluchová, Gabriel Török
Title: Computer simulation of trajectories of particles affected by general relativistic Poynting-Robertson effect
We developed a computer code which numerically integrates relativistic trajectories of particles in strong gravitational fileds of neutron stars under an influence of radiation pressure. Developed numerical simulation allows to model changes of density, matter distribution and other parametes of accretion discs according to the luminosity of central radiating stellar object.
Author: John Miller (University of Oxford)
Title: Horizon formation during gravitational collapse - an update
Author: Zdeněk Stuchlík (Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava, Czech Republic)
Title: General relativistic polytropes
Author: Bobir Toshmatov (Research Centre for Theoretical Physics and Astrophysics, Institute of Physics, Silesian University in Opava, Czech Republic)
Title: Quasinormal modes of black hole in the braneworld
Author: Gabriel Török (Research Centre for Computational Physics and Data Analysis, Institute of Physics, Silesian University in Opava, Czech Republic)
Co-authors: Kateřina Goluchová, Martin Urbanec, Eva Šrámková, Karel Adámek, Gabriela Urbancová, Tomáš Pecháček, Pavel Bakala, Zdeněk Stuchlík, Jiří Horák, Jakub Juryšek
Title: Models of Twin Peak Quasi-periodic Oscillations vs. Realistic Neutron Star Equation of State (QPO Lecture I)
Twin-peak quasi-periodic oscillations (QPOs) are observed in the X-ray power-density spectra of several accreting low-mass neutron star (NS) binaries. We confront QPO models with various NS equations of state (EoS). The application of concrete EoS together with a particular QPO model yields a specific mass–angular-momentum relation. This degeneracy in mass and angular momentum can be removed when the NS spin frequency inferred from the X-ray burst observations is considered. We inspect a large set of EoS and find that several EoS are not compatible with any of the considered QPO models. We conclude that when the NS spin frequency in 4U 1636-53 is close to 580Hz we can exclude 51 from 90 of the considered combinations of EoS and QPO models. We also discuss additional restrictions that may exclude even more combinations. Namely, there are 13 EOS compatible with the observed twin peak QPOs and the relativistic precession model. However, when considering the low frequency QPOs and Lense-Thirring precession, only 5 EOS are compatible with the model.
Author: Gabriel Török (Research Centre for Computational Physics and Data Analysis, Institute of Physics, Silesian University in Opava, Czech Republic)
Co-authors: Kateřina Goluchová, Jiří Horák, Eva Šrámková, Martin Urbanec, Tomáš Pecháček, Pavel Bakala
Title: Twin peak quasi-periodic oscillations as signature of oscillating cusp torus (QPO Lecture II)
Serious theoretical effort has been devoted to explain the observed frequencies of twin-peak quasi-periodic oscillations (HF QPOs) observed in low-mass X-ray neutron star binaries. Here we propose a new model of HF QPOs. Within its framework we consider an oscillating torus with cusp that changes location r0 of its centre around radii very close to innermost stable circular orbit. The observed variability is assigned to global modes of accreted fluid motion that may give strong modulation of both accretion disc radiation and the accretion rate. For a given space-time geometry, the model predicts that QPO frequencies are function of single parameter r0. We illustrate that the model can provide fits of data comparable to those reached by other models, or even better. In particular, it is compared to relativistic precession model. Moreover, we also illustrate that the model consideration is compatible with consideration of models of a rotating neutron star in the atoll source 4U 1636-53.
Author: Gabriel Urbancová (Astronomical Institute, Czech Academy of Sciences, Czech Republic; Research Centre for Computational Physics and Data Analysis, Institute of Physics, Silesian University in Opava, Czech Republic)
Co-authors: Vladimír Karas, Michal Dovčiak, Martin Urbanec, Gabriel Török
Title: An XSPEC model to explore spectral features from black-hole sources - III. The effects of Hartle-Thorne metric
Classical General Relativity describes stationary, isolated mathematical black holes by only four parameters - mass, spin, electric charge, and magnetic charge (where both charges are thought to vanish in astrophysically realistic circumstances). Cosmic black holes, however, are influenced by additional processes, so the resulting gravitational field may be different from the text-book (Kerr) solution. In order to develop a well-defined scheme and to test the additional moments via the emerging radiation, we have embarked on a project to include the effects of Hartle-Thorne metric in the KY spectral fitting tool, which so far has allowed to consider only the first two parameters of the Kerr metric.