1,496 research outputs found
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.Comment: 12 pages, requires aasms.sty, to appear in The Astrophysical Journal,
Vol. 470 (October 20, 1996), figures available upon request from the Author,
or at http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
Strong-gravity effects acting on polarization from orbiting spots
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.Comment: 6 pages, 2 figures; proceedings of "The Coming of Age of X-ray
Polarimetry," Rome, Italy, April 27-30, 200
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.Comment: Letter to MNRAS, 5 pages and 3 figures; also available at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
The Galactic Center
In the past decade high resolution measurements in the infrared employing
adaptive optics imaging on 10m telescopes have allowed determining the three
dimensional orbits stars within ten light hours of the compact radio source at
the center of the Milky Way. These observations show the presence of a three
million solar mass black hole in Sagittarius A* beyond any reasonable doubt.
The Galactic Center thus constitutes the best astrophysical evidence for the
existence of black holes which have long been postulated, and is also an ideal
`lab' for studying the physics in the vicinity of such an object. Remarkably,
young massive stars are present there and probably have formed in the innermost
stellar cusp. Variable infrared and X-ray emission from Sagittarius A* are a
new probe of the physical processes and space-time curvature just outside the
event horizon.Comment: Write up of the talk at IAU Symposium No. 238 (21-25 August 2006,
Prague), to appear in Proceedings of "Black Holes: from Stars to Galaxies"
(Cambridge University Press), p. 17
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.Comment: 12 pages, to appear in Publications of the Astronomical Society of
Japan, Vol. 48 (October 1996), figures available upon request from the
authors, or at
http://otokar.troja.mff.cuni.cz/user/karas/au_www/karas/papers.ht
Polarization of light from warm clouds above an accretion disk: effects of strong-gravity near a black hole
We study polarization from scattering of light on a cloud in radial motion
along the symmetry axis of an accretion disk. Radiation drag from the disk and
gravitational attraction of the central black hole are taken into account, as
well as the effect of the cloud cooling in the radiation field. This provides
us with a self-consistent toy-model for predicted lightcurves, including the
linear polarization that arises from the scattering. Strong gravitational
lensing creates indirect images; these are formed by photons that originate
from the disk, get backscattered onto the photon circular orbit and eventually
redirected towards an observer. Under suitable geometrical conditions the
indirect photons may visibly influence the resulting magnitude of polarization
and light-curve profiles. Relevant targets are black holes in active galactic
nuclei and stellar-mass Galactic black-holes exhibiting episodic
accretion/ejection events.Comment: Accepted for publication in PASJ; 7 pages, 4 figure
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