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Search Results: 1 - 10 of 130409 matches for " V. Karas "
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Fourier-phase analysis of the orbiting bright-spot model for AGN variability
V. Karas
Physics , 1997, DOI: 10.1093/mnras/288.1.12
Abstract: 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.
Retro-lensing light curves near a black hole
V. Karas
Physics , 2009,
Abstract: We model the light-curves from radiation-driven clouds near an accreting black hole. Taking into account the multiple images due to strong gravitational lensing, we find that sharp spikes can significantly enhance the observed flux. Following our previous work (Horak & Karas 2006a,b) we assume that scattering of ambient light takes place in a cloud that is in radial motion under a combined influence of black hole gravity and the radiation field. The retro-lensed photons give rise to peaks in the observed signal that follow with a characteristic time lag after the direct-image photons. Duration of these features is very short and the predicted polarization varies abruptly on the time-scale comparable with the light-crossing time of the system -- a signature of the photon orbit. We also consider the polarization properties of scattered light.
Strong-gravity effects acting on polarization from orbiting spots
V. Karas
Physics , 2009,
Abstract: Accretion onto black holes often proceeds via an accretion disc or a temporary disc-like pattern. Variability features, observed in the light curves of such objects, and theoretical models of accretion flows suggest that accretion discs are inhomogeneous and non-axisymmetric. Fast orbital motion of the individual clumps can modulate the observed signal. If the emission from these clumps is partially polarized, which is likely the case, then rapid polarization changes of the observed signal are expected as a result of general relativity effects.
Light curve of a source orbiting around a black hole: A fitting-formula
V. Karas
Physics , 1996, DOI: 10.1086/177904
Abstract: 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.
Magnetic Neutral Points and Electric Lines of Force in Strong Gravity of a Rotating Black Hole  [PDF]
V. Karas, O. Kopá?ek, D. Kunneriath
International Journal of Astronomy and Astrophysics (IJAA) , 2013, DOI: 10.4236/ijaa.2013.33A003
Abstract:

Magnetic field can be amplified and twisted near a supermassive black hole residing in a galactic nucleus. At the same time magnetic null points develop near the horizon. We examine a large-scale oblique magnetic field near a rotating (Kerr) black hole as an origin of magnetic layers, where the field direction changes abruptly in the ergosphere region. In consequence of this, magnetic null points can develop by purely geometrical effects of the strong gravitational field and the frame-dragging mechanism. We identify magnetic nulls as possible sites of magnetic reconnection and suggest that particles may be accelerated efficiently by the electric component. The situation we discuss is relevant for starving nuclei of some galaxies which exhibit episodic accretion events, namely, Sagittarius A* black hole in our Galaxy.

Enhancing the rate of tidal disruptions of stars by a self-gravitating disc around a massive central black hole
Karas V.,?ubr L.
EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20123901003
Abstract: We further study the idea that a self-gravitating accretion disc around a supermassive black hole can increase the rate of gradual orbital decay of stellar trajectories (and hence tidal disruption events) by setting some stars on eccentric trajectories. Cooperation between the gravitational field of the disc and the dissipative environment can provide a mechanism explaining the origin of stars that become bound tightly to the central black hole. We examine this process as a function of the black hole mass and conclude that it is most efficient for intermediate central masses of the order of ~ 104M . Members of the cluster experience the stage of orbital decay via collisions with an accretion disc and by other dissipative processes, such as tidal effects, dynamical friction and the emission of gravitational waves. Our attention is concentrated on the region of gravitational dominance of the central body. Mutual interaction between stars and the surrounding environment establishes a non-spherical shape and anisotropy of the nuclear cluster. In some cases, the stellar sub-system acquires ring-type geometry. Stars of the nuclear cluster undergo a tidal disruption event as they plunge below the tidal radius of the supermassive black hole.
Pseudo-Newtonian models of a rotating black hole field
O. Semerak,V. Karas
Physics , 1999,
Abstract: Several topics are discussed regarding the pseudo-Newtonian models of the gravitational field around rotating black holes
Doppler tomography of relativistic accretion disks
V. Karas,P. Kraus
Physics , 1996, DOI: 10.1093/pasj/48.5.771
Abstract: 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.
Motion of a Solar Mass Compact Object Around a Massive Rotating Black Hole
V. Karas,D. Vokrouhlicky
Physics , 1993,
Abstract: 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.
Region of magnetic dominance near a rotating black hole
V. Karas,M. Dovciak
Physics , 1997, DOI: 10.1023/A:1018872625775
Abstract: 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.
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