675 research outputs found
GR models of the X-ray spectral variability of MCG--6-30-15
We study in detail the GR models of the X-ray spectral variability for
various geometries of the X-ray source and with various relativistic effects
being the dominant cause of spectral variability. The predicted properties are
compared with the Suzaku observational data of the Seyfert 1 galaxy
MCG--6-30-15. The data disfavor models with the X-ray source (1) moving
vertically on the symmetry axis or (2) corotating with the disc and changing
height not far above the disc surface. The most likely explanation for the
observed variability is given by the model involving the X-ray source located
at a very small, varying distance from a rapidly rotating black hole. This
model predicts some enhanced variations in the red wing of the Fe line, which
are not seen in the Suzaku observations. However, the enhanced variability of
the red wing, while ruled out by the Suzaku data, is consistent with an excess
RMS variability, between 5 and 6 keV, reported for some previous ASCA and XMM
observations. We speculate that the presence or lack of such a feature is
related to the change of the ionization state of the innermost part of the
disc, however, investigation of such effects is currently not possible in our
model (where a neutral disc is assumed). If the model, completed by description
of ionization effects, proves to be fully consistent with the observational
data, it will provide a strong indication that the central black hole in
MCG--6-30-15 rotates rapidly, supporting similar conclusions derived from the
Fe line profile.Comment: 15 pages, accepted for publication in A&
Bulk motion Comptonization in black-hole accretion flows
We study spectra generated by Comptonization of soft photons by cold
electrons radially free-falling onto a black hole. We use a Monte Carlo method
involving a fully relativistic description of Comptonization in the Kerr
space-time. In agreement with previous studies, we find that Comptonization on
the bulk motion of free fall gives rise to power-law spectra with the photon
index of Gamma >~ 3. In contrast to some previous studies, we find that these
power-law spectra extend only to energies << 511 keV. We indicate several
effects resulting in generic cutoffs of such spectra at several tens of keV,
regardless of any specific values of physical parameters in the model. This
inefficiency of producing photons with energies > 100 keV rules out bulk motion
Comptonization as a main radiative process in soft spectral states of
black-hole binaries. The normalization of the power law (below the cutoff) with
respect to the peak of the blackbody emission of the surrounding disc is
typically very low, except for models with an overlap between the disc and the
plasma, in which case the spectra are very soft, Gamma >~ 4.Comment: 10 pages, 4 figures, revised version, accepted for publication in
MNRA
Modelling the energy dependencies of high-frequency QPO in black hole X-ray binaries
We model energy dependencies of the quasi periodic oscillations (QPO) in the
model of disc epicyclic motions, with X-ray modulation caused by varying
relativistic effects. The model was proposed to explain the high frequency QPO
observed in X-ray binaries. We consider two specific scenarios for the geometry
of accretion flow and spectral formation. Firstly, a standard cold accretion
disc with an active X-ray emitting corona is assumed to oscillate. Secondly,
only a hot X-ray emitting accretion flow oscillates, while the cold disc is
absent at the QPO radius. We find that the QPO spectra are generally similar to
the spectrum of radiation emitted at the QPO radius, and they are broadened by
the relativistic effects. In particular, the QPO spectrum contains the disc
component in the oscillating disc with a corona scenario. We also review the
available data on energy dependencies of high frequency QPO, and we point out
that they appear to lack the disc component in their energy spectra. This would
suggest the hot flow geometry in the spectral states when high frequency QPO
are observed.Comment: 8 pages, accepted for publication in MNRA
On the light-bending model of X-ray variability of MCG-6-30-15
We apply the light bending model of X-ray variability to Suzaku data of the
Seyfert 1 galaxy MCG-6-30-15. We analyze the energy dependence of the root mean
square (rms) variability, and discuss conditions necessary for the model to
explain the characteristic decrease of the source variability around 5-8 keV. A
model, where the X-ray source moves radially rather than vertically close to
the disk surface, can indeed reproduce the reduced variability near the energy
of the Fe Kalpha line, although the formal fit quality is poor. The model then
predicts the energy spectra, which can be compared to observational data. The
spectra are strongly reflection dominated, and do not provide a good fit to
Suzaku spectral data of the source. The inconsistency of this result with some
previous claims can be traced to our using data in a broader energy band, where
effects of warm absorber in the spectrum cannot be neglected.Comment: 6 pages, PASJ, accepte
On the influence of relativistic effects on X-ray variability of accreting black holes
X-rays produced by compact flares co-rotating with a Keplerian accretion disc
are modulated in time by Doppler effects. We improve on previous calculations
of these effects by considering recent models of intrinsic X-ray variability,
and compute the expected strength of the relativistic signal in current data of
Seyfert galaxies and black hole binaries. Such signals could clearly be seen
in, for example, recent XMM-Newton data from MCG-6-30-15, if indeed the X-rays
were produced by co-rotating flares concentrated toward the inner disc edge
around an extreme Kerr black hole. Lack of the signal in the data collected so
far gives support to models, where the X-ray sources in active galaxies do not
follow Keplerian orbits close to the black hole.Comment: 8 pages, replaced to match the version accepted by MNRAS. Extended
discussion of assumptions on relation between time-scale of a flare and its
radial position, and their influence on results. Conclusions unchange
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