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V. Karas

Publications and source records attributed to V. Karas.

160 records · Page 9Linked to original sources
arXiv2000

Magnetic fields around black holes

The electromagnetic field near a rotating black hole is being explored in educational style here. By employing analytic solutions for electrovacuum fields, we plot the surfaces of constant flux and we show how the field is dragged around the black hole by purely geometrical effects of the strong gravitational field. We visualize the structure of magnetic lines and we also propose possible astrophysical applications of more realistic situations involving the presence of plasma: The entangled and twisted field lines result in reconnection processes which accelerate the particles injected into the vicinity of the black hole. Such acceleration mechanisms of the electromagnetic origin are considered as a possible source of high-speed outflows emerging from numerous nuclei of galaxies where black holes reside.

astro-ph↗
arXiv1999

Effects of Kerr Spacetime on Spectral Features from X-Ray Illuminated Accretion Discs

We performed detailed calculations of the relativistic effects acting on both the reflection continuum and the iron line from accretion discs around rotating black holes. Fully relativistic transfer of both illuminating and reprocessed photons has been considered in Kerr spacetime. We calculated overall spectra, line profiles and integral quantities, and present their dependences on the black hole angular momentum. We show that the observed EW of the lines is substantially enlarged when the black hole rotates rapidly and/or the source of illumination is near above the hole. Therefore, such calculations provide a way to distinguish among different models of the central source.

astro-ph↗
arXiv1999

An orbiter crossing an accretion disc

We further investigate the long-term evolution of a trajectory of a stellar-mass orbiter which is gravitationally bound to a massive central body. The orbiter undergoes repetitionary collisions with an accretion disc. We consider eccentric orbits intersecting the disc once or twice per each revolution, and we solve the equations for osculating elements. We find terminal radii of the orbits and time needed to bring the orbit into the disc plane as function of initial parameters, and we show that previous simplified estimates (derived for the case of low eccentricity) remain valid within factor of two. The discussion presented in this paper offers a toy model of the orbital evolution of a satellite which passes through the rarefied gaseous environment of a stationary disc. We demonstrate that the drag on the satellite should be taken into account in calculations of the stellar distribution near the super-massive black hole in galactic centers. On the other hand, it does not impose serious restrictions on the results of gravitational wave experiments, except for the case of discs with rather high surface density.

astro-ph↗
arXiv1998

Stellar dynamics in a galactic centre surrounded by a massive accretion disc. I. Newtonian description

The long-term evolution of stellar orbits bound to a massive centre is studied in order to understand the cores of star clusters in central regions of galaxies. Stellar trajectories undergo tiny perturbation, the origin of which is twofold: (i) gravitational field of a thin gaseous disc surrounding the galactic centre, and (ii) cumulative drag due to successive interactions of the stars with material of the disc. Both effects are closely related because they depend on the total mass of the disc, assumed to be a small fraction of the central mass. It is shown that, in contrast to previous works, most of the retrograde (with respect to the disc) orbits are captured by the central object, presumably a massive black hole. Initially prograde orbits are also affected, so that statistical properties of the central star cluster in quasi-equilibrium may differ significantly from those deduced in previous analyses.

astro-ph↗
arXiv1998

A method to determine the parameters of black holes in AGNs and galactic X-ray sources with periodic modulation of variability

We propose a simple and unambiguous way to deduce the parameters of black holes which may reside in AGNs and some types of X-ray binaries. The black-hole mass and angular momentum are determined in physical units. The method is applicable to the sources with periodic components of variability, provided one can assume the following: (i) Variability is due to a star or a stellar-mass compact object orbiting the central black hole and passing periodically through an equatorial accretion disk (variability time-scale is given by the orbital period). (ii) The star orbits almost freely, deviation of its trajectory due to passages through the disk being very weak (secular); the effect of the star on the disk, on the other hand, is strong enough to yield observable photometric and spectroscopic features. (iii) The gravitational field within the nucleus is that of the (Kerr) black hole, the star and the disk contribute negligibly.

astro-ph↗
arXiv1997

On the runaway instability of relativistic tori

We further investigate the runaway instability of relativistic tori around black holes assuming a power law distribution of the specific angular momentum. We neglect the self-gravity of the torus; thus the gravitational field of the system is determined by the central rotating black hole alone (the Kerr geometry). We find that for any positive (i.e. consistent with the local Rayleigh stability condition) power law index of the angular momentum distribution, the tori are always runaway stable for a sufficiently large spin of the black hole. However, before concluding that some (astrophysically realistic) tori could indeed be runaway stable, one should include, in full general relativity, the destabilizing effect of self-gravity.

astro-ph↗
arXiv1997

Stellar capture by an accretion disc

Long-term evolution of a stellar orbit captured by a massive galactic center via successive interactions with an accretion disc has been examined. An analytical solution describing evolution of the stellar orbital parameters during the initial stage of the capture was found. Our results are applicable to thin Keplerian discs with an arbitrary radial distribution of density and rather general prescription for the star-disc interaction. Temporal evolution is given in the form of quadrature which can be carried out numerically.

astro-ph↗
arXiv1997

Region of magnetic dominance near a rotating black hole

This is a brief contribution in which a simplified criterion of the relevance of the test-particle approximation describing motion of material near a magnetized black hole is discussed. Application to processes of the dissipative collimation of astronomical jets (as proposed by de Felice and Curir, 1992) is mentioned.

gr-qc↗
arXiv1997

Fourier-phase analysis of the orbiting bright-spot model for AGN variability

Fourier power spectra and phases of a signal from a large number of radiating sources orbiting around a black hole are investigated. It is assumed that the individual sources (bright spots) are located in an accretion disc and their lifetime exceeds the corresponding orbital period. This model is relevant for the short-time X-ray variability of active galactic nuclei. Previous works on this subject were mostly concentrated on temporal characteristics and power spectra of observed light curves. In our present contribution, Fourier phases are brought into consideration and studied systematically for a broad range of input parameters. In particular, conditions for the phase coherence are discussed. It is shown that one can discriminate between the two classes of models which are currently under consideration--orbital motion of a large number of sources versus short-lived independent flares--although parameters of the model are not completely arbitrary. It is also shown that predicted power spectra depend rather strongly on the spot distribution across the disk surface. We conclude that the orbital motion of the spots cannot be the only reason for the source fluctuations, but it certainly influences observational properties of the source intrinsic variability.

astro-ph↗
arXiv1996

Doppler tomography of relativistic accretion disks

Spectral lines from a source orbiting around a compact object are studied. Time variations of observed frequency and count rate due to motion of the source and gravitational lensing are considered. Gravitational field of the central object is described by the Kerr metric. It is shown that: (i) simultaneous temporal and frequency resolution enables us to restrict parameters of the model (inclination angle, position of the source, angular momentum of the black hole); (ii) techniques of image restoration, familiar from other fields of astronomy, can be applied to study inner regions of active galactic nuclei. This contribution is relevant for extremely variable X-ray sources with high parameter of efficiency, such as Seyfert 1 galaxy PHL 1092 observed by ROSAT.

astro-ph↗
arXiv1996

Light curve of a source orbiting around a black hole: A fitting-formula

A simple, analytical fitting-formula for a photometric light curve of a source of light orbiting around a black hole is presented. The formula is applicable for sources on a circular orbit with radius smaller than 45 gravitational radii from the black hole. This range of radii requires gravitational focusation of light rays and the Doppler effect to be taken into account with care. The fitting-formula is therefore useful for modelling the X-ray variability of inner regions in active galactic nuclei.

astro-ph↗
arXiv1994

Emission Line Profiles from Self-Gravitating Thin Disks

We have constructed general relativistic models of a stationary, axially symmnetric, Keplerian thin disk around a rotating black hole. We computed profiles of a spectral line, emitted in the inner region of the disk. In our models we have taken into account also the self-gravity of the disk. The aim of this work is to study gravitational effects on the line profiles in connection with the X-ray features observed in spectra of active galactic nuclei. In some cases, the calculated profiles are clearly affected by the disk gravity but relativistic dragging effects are found to be negligible.

astro-ph↗
arXiv1993

Motion of a Solar Mass Compact Object Around a Massive Rotating Black Hole

This preprint is based on two articles accepted by MNRAS and ApJ. Our aim is to study the evolution of the orbit of a star under the influence of interactions with an accretion disc in an AGN. The model considered consists of a low-mass compact object orbiting a supermassive black hole and colliding periodically with the accretion disc. Gravitational field of the nucleus is described by the Kerr metric. The star is assumed to move along a geodesic arc between successive interactions with an equatorial accretion disc. We study the gravitomagnetic (Lense-Thirring) precession of the trajectory. We do not assume any particular value for the eccentricity or inclination of the orbit or the angular momentum of the black hole. We also discuss the periodicity related to the relativistic shift of the pericenter. The model of star-disc interactions has been suggested to explain the X-ray variability observed in the Seyfert galaxy NGC~6814. We briefly discuss on this subject.

astro-ph↗
arXiv1993

Similarity of the variability patterns in the Exosat and Ginga folded light curves of the Seyfert galaxy NGC 6814

The Seyfert galaxy NGC 6814 is known to show periodic variation of its X-ray luminosity. We found that the sequences of peaks (variability patterns) in the folded X-ray light curves constructed from the {\it Exosat} and {\it Ginga} data are remarkably similar when one ignores amplitudes of the peaks and considers only their phases. The stable pattern consists of five peaks which are present in the both curves. The phases of the corresponding peaks coincide with an accuracy of about 10 degrees. The probability that this coincidence occurs by chance is less than about 1\% according to the most conservative estimate. The observed stable pattern of peaks may be produced by a stable distribution of ``bright spots'' on the accretion disk surface, {\it e.g.} by strong vortices or magnetic flux tubes.

astro-ph↗