329 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
Tidal Capture by a Black Hole and Flares in Galactic Centres
We present the telltale signature of the tidal capture and disruption of an
object by a massive black hole in a galactic centre. As a result of the
interaction with the black hole's strong gravitational field, the object's
light curve can flare-up with characteristic time of the order of 100 sec
\times (M_{bh} / 10^6 M_{Solar}). Our simulations show that general relativity
plays a crucial role in the late stages of the encounter in two ways: (i) due
to the precession of perihelion, tidal disruption is more severe, and (ii)
light bending and aberration of light produce and enhance flares seen by a
distant observer. We present our results for the case of a tidally disrupted
Solar-type star. We also discuss the two strongest flares that have been
observed at the Galactic centre. Although the first was observed in X-rays and
the second in infra-red, they have almost identical light curves and we find it
interesting that it is possible to fit the infra-red flare with a rather simple
model of the tidally disrupted comet-like or planetary object. We discuss the
model and possible scenarios how such an event can occur.Comment: 3 pages, 1 figur
Modelling the light-curves of objects tidally disrupted by a black hole
Tidal disruption by massive black holes is a phenomenon, during which a large
part of gravitational energy can be released on a very short time-scale. The
time-scales and energies involved during X-ray and IR flares observed in
Galactic centre suggest that they may be related to tidal disruption events.
Furthermore, aftermath of a tidal disruption of a star by super-massive black
hole has been observed in some galaxies, e.g. RX J1242.6-1119A. All these
discoveries increased the demand for tools for tidal disruption study in curved
space-time. Here we summarise our study of general relativistic effects on
tidal deformation of stars and compact objects.Comment: 2 pages, to appear in the proceedings of the JENAM 2008, Symposium 7:
"Grand Challenges in Computational Astrophysics
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
The Automatic Real-Time GRB Pipeline of the 2-m Liverpool Telescope
The 2-m Liverpool Telescope (LT), owned by Liverpool John Moores University,
is located in La Palma (Canary Islands) and operates in fully robotic mode. In
2005, the LT began conducting an automatic GRB follow-up program. On receiving
an automatic GRB alert from a Gamma-Ray Observatory (Swift, INTEGRAL, HETE-II,
IPN) the LT initiates a special override mode that conducts follow-up
observations within 2-3 min of the GRB onset. This follow-up procedure begins
with an initial sequence of short (10-s) exposures acquired through an r' band
filter. These images are reduced, analyzed and interpreted automatically using
pipeline software developed by our team called "LT-TRAP" (Liverpool Telescope
Transient Rapid Analysis Pipeline); the automatic detection and successful
identification of an unknown and potentially fading optical transient triggers
a subsequent multi-color imaging sequence. In the case of a candidate brighter
than r'=15, either a polarimetric (from 2006) or a spectroscopic observation
(from 2007) will be triggered on the LT. If no candidate is identified, the
telescope continues to obtain z', r' and i' band imaging with increasingly
longer exposure times. Here we present a detailed description of the LT-TRAP
and briefly discuss the illustrative case of the afterglow of GRB 050502a,
whose automatic identification by the LT just 3 min after the GRB, led to the
acquisition of the first early-time (< 1 hr) multi-color light curve of a GRB
afterglow.Comment: PASP, accepted (8 pages, 3 figures
The GRB Variability/Peak Luminosity Correlation: new results
We report test results of the correlation between time variability and peak
luminosity of Gamma-Ray Bursts (GRBs), using a larger sample (32) of GRBs with
known redshift than that available to Reichart et al. (2001), and using as
variability measure that introduced by these authors. The results are puzzling.
Assuming an isotropic-equivalent peak luminosity, as done by Reichart et al.
(2001), a correlation is still found, but it is less relevant, and inconsistent
with a power law as previously reported. Assuming as peak luminosity that
corrected for GRB beaming for a subset of 16 GRBs with known beaming angle, the
correlation becomes little less significant.Comment: 11 pages, 10 figures, MNRAS, accepte
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