152 research outputs found
Clumps of material orbiting a black hole and the QPOs
Clumps of material orbiting a black hole may be disturbed, somewhat like
comets in the Kuiper belt, to relatively small periastron orbits. Each
periastron passage changes the orbital parameters in such a way that the orbit
becomes more and more eccentric and the angular momentum approaches the
critical value for tidal capture. When this value is reached, the body is
suddenly caught by the relativistic potential to the last periastron (occurring
at two Schwarzschild radii for a non rotating black hole). In this process the
transfer of orbital into internal energy heats the body before it makes a few
more turns toward the horizon of the black hole. Because of strong relativistic
effects this last bright message from the object is seen as a quasi-periodic
flare. Assuming that a black hole may be fed by a large number of such small
debris we calculate light curves expected from such events. We investigate the
resemblance of the Fourier spectra of such light curves with those of observed
QPOs.Comment: 3 pages, 6 figure
Do flares in Sagittarius A* reflect the last stage of tidal capture?
In recent years the case for the presence of 3-4 10^6 M_sun black hole in our
Galactic Center has gained strength from results of stellar dynamics
observations and from the detection of several rapid X-ray and IR flares
observed in the Sagittarius A* from 2000 to 2004. Here we explore the idea that
such flares are produced when the central black hole tidally captures and
disrupts a small body - e.g. a comet or an asteroid.Comment: 6 pages, 9 figures, acknowledgments added, to appear in the
Proceedings of the Albert Einstein's Century International Conference, Paris
200
Relativistic Effects on the Appearance of a Clothed Black Hole
For an accretion disk around a black hole, the strong relativistic effects
affect every aspect of the radiation from the disk, including its spectrum,
light-curve, and image. This work investigates in detail how the images of a
thin disk around a black hole will be distorted, and what the observer will see
from different viewing angles and in different energy bands.Comment: 4 pages, 5 figures. Based on the poster presented at the Sixth
Pacific Rim Conference on Stellar Astrophysics (Xi'an, China, July 11-17,
2002). Color versions of figures are given separatel
Tidal effects on small bodies by massive black holes
The compact radio source Sagittarius A (Sgr A) at the centre of our Galaxy
harbours a supermassive black hole, whose mass has been measured from stellar
orbital motions. Sgr A is therefore the nearest laboratory where super-massive
black hole astrophysics can be tested, and the environment of black holes can
be investigated. Since it is not an active galactic nucleus, it also offers the
possibility of observing the capture of small objects that may orbit the
central black hole. We study the effects of the strong gravitational field of
the black hole on small objects, such as a comet or an asteroid. We also
explore the idea that the flares detected in Sgr A might be produced by the
final accretion of single, dense objects with mass of the order of 10^20 g, and
that their timing is not a characteristic of the sources, but rather of the
space-time of the central galactic black hole in which they are moving. We find
that tidal effects are strong enough to melt the solid object, and present
calculations of the temporal evolution of the light curve of infalling objects
as a function of various parameters. Our modelling of tidal disruption suggests
that during tidal squeezing, the conditions for synchrotron radiation can be
met. We show that the light curve of a flare can be deduced from dynamical
properties of geodesic orbits around black holes and that it depends only
weakly on the physical properties of the source.Comment: 10 pages, 14 figures, A&A accepte
Strong deflection limit of black hole gravitational lensing with arbitrary source distances
The gravitational field of supermassive black holes is able to strongly bend
light rays emitted by nearby sources. When the deflection angle exceeds ,
gravitational lensing can be analytically approximated by the so-called strong
deflection limit. In this paper we remove the conventional assumption of
sources very far from the black hole, considering the distance of the source as
an additional parameter in the lensing problem to be treated exactly. We find
expressions for critical curves, caustics and all lensing observables valid for
any position of the source up to the horizon. After analyzing the spherically
symmetric case we focus on the Kerr black hole, for which we present an
analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review
The influence of solar spectral lines on electron concentration in terrestrial ionosphere
One of the methods of detection and analysis of solar flares is observing the
time variations of certain solar spectral lines. During solar flares, a raise
of electron concentration occurs in Earth's ionosphere which results in
amplitude and phase variations of the recorded very low frequency (VLF) waves.
We compared the data obtained by the analysis of recorded VLF signals and line
spectra for different solar flares. In this paper we treated the DHO VLF signal
transmitted from Germany at the frequency of 23.4 kHz recorded by the AWESOME
system in Belgrade (Serbia) during solar flares in the period between 10:40 UT
and 13:00 UT on 2011 April 22
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