985 research outputs found
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
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.Comment: 26 pages, 8 figures, uuencoded postscript file, to appear in The
Astrophysical Journal, Part I. Printed version available upon request from
the author
Frame-dragging effects on magnetic fields near a rotating black hole
We discuss the role of general relativity frame dragging acting on magnetic
field lines near a rotating (Kerr) black hole. Near ergosphere the magnetic
structure becomes strongly influenced and magnetic null points can develop. We
consider aligned magnetic fields as well as fields inclined with respect to the
rotation axis, and the two cases are shown to behave in profoundly different
ways. Further, we construct surfaces of equal values of local electric and
magnetic intensities, which have not yet been discussed in the full generality
of a boosted rotating black hole.Comment: to appear in the proceedings of "The Central Kiloparsec in Galactic
Nuclei (AHAR 2011)", Journal of Physics: Conference Series (JPCS), IOP
Publishin
Interpreting the High Frequency QPO Power Spectra of Accreting Black Holes
In the context of a relativistic hot spot model, we investigate different
physical mechanisms to explain the behavior of quasi-periodic oscillations
(QPOs) from accreting black holes. The locations and amplitudes of the QPO
peaks are determined by the ray-tracing calculations presented in Schnittman &
Bertschinger (2004a): the black hole mass and angular momentum give the
geodesic coordinate frequencies, while the disk inclination and the hot spot
size, shape, and overbrightness give the amplitudes of the different peaks. In
this paper additional features are added to the existing model to explain the
broadening of the QPO peaks as well as the damping of higher frequency
harmonics in the power spectrum. We present a number of analytic results that
closely agree with more detailed numerical calculations. Four primary pieces
are developed: the addition of multiple hot spots with random phases, a finite
width in the distribution of geodesic orbits, Poisson sampling of the detected
photons, and the scattering of photons from the hot spot through a corona of
hot electrons around the black hole. Finally, the complete model is used to fit
the observed power spectra of both type A and type B QPOs seen in XTE
J1550-564, giving confidence limits on each of the model parameters.Comment: 30 pages, 5 figures, submitted to Ap
Reflection nebulae in the Galactic Center: the case for soft X-ray imaging polarimetry
The origin of irradiation and fluorescence of the 6.4 keV bright giant
molecular clouds surrounding Sgr A*, the central supermassive black hole of our
Galaxy, remains enigmatic. Testing the theory of a past active period of Sgr A*
requires X-ray polarimetry. In this paper, we show how modern imaging
polarimeters could revolutionize our understanding of the Galactic Center.
Through Monte Carlo modeling, we produce a 4-8 keV polarization map of the
Galactic Center, focusing on the polarimetric signature produced by Sgr B1, Sgr
B2, G0.11-0.11, Bridge E, Bridge D, Bridge B2, MC2, MC1, Sgr C3, Sgr C2, and
Sgr C1. We estimate the resulting polarization, include polarized flux dilution
by the diffuse plasma emission detected toward the GC, and simulate the
polarization map that modern polarimetric detectors would obtain assuming the
performances of a mission prototype. The eleven reflection nebulae investigated
in this paper present a variety of polarization signatures, ranging from nearly
unpolarized to highly polarized (about 77%) fluxes. A major improvement in our
simulation is the addition of a diffuse, unpolarized plasma emission that
strongly impacts soft X-ray polarized fluxes. The dilution factor is in the
range 50% - 70%, making the observation of the Bridge structure unlikely even
in the context of modern polarimetry. The best targets are the Sgr B and Sgr C
complexes, and the G0.11-0.11 cloud. An exploratory observation of a few
hundred kilo-seconds of the Sgr B complex would allow a significant detection
of the polarization and be sufficient to derive hints on the primary source of
radiation. A more ambitious program (few Ms) of mapping the giant molecular
clouds could then be carried out to probe with great precision the turbulent
history of Sgr A*, and place important constraints on the composition and
three-dimensional position of the surrounding gas.Comment: 7 pages, 3 figures, 2 tables, accepted for publication in A&
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