48 research outputs found
Testing a model of variability of X-ray reprocessing features in Active Galactic Nuclei
A number of recent results from X-ray observations of Active Galactic Nuclei
involving the Fe K alpha line (reduction of line variability compared to the
X-ray continuum variability, the X-ray ``Baldwin effect'') were attributed to a
presence of a hot, ionized skin of an accretion disc, suppressing emission of
the line. The ionized skin appears as a result of the thermal instability of
X-ray irradiated plasma. We test this hypothesis by computing the Thomson
thickness of the hot skin on top of the 'alpha P_tot' Shakura-Sunyaev disc, by
simultaneously solving the vertical structure of both the hot skin and the
disc. We then compute a number of relations between observable quantities, e.g.
the hard X-ray flux, amplitude of the observed reprocessed component,
relativistic smearing of the K alpha line, the r.m.s. variability of the hard
X-rays. These relations can be compared to present and future observations. We
point out that this mechanism is unlikely to explain the behaviour of the X-ray
source in MCG-6-30-15, where there is a number of arguments against the
existence of a thick hot skin, but it can work for some other Seyfert 1
galaxies.Comment: 12 pages. MNRAS, accepte
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
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 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
Vertical Structure of Accretion Discs with Hot Coronae in AGN
We study vertical structure of radiation pressure dominated disc with a hot
corona. We include all the relevant processes like bound--free opacity and
convection. We show that the presence of the corona modifies considerably the
density and the opacity of the disc surface layers which are important from the
point of view of spectrum formation. The surface of the disc with a corona is
much denser and less ionized than the surface of a bare disc. Such a disc is
likely to produce a neutral reflection and a local spectrum close to a black
body. This effect will help to reconcile the predictions of accretion disc
models with the observational data since a neutral reflection and a lack of
Lyman edge are generally seen in AGN.Comment: 10 pages, 9 figures, submitted to MNRA
Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
We analyze a large collection of RXTE archive data from April 1997 to August
2003 of the bright X-ray source Scorpius X-1 in order to study the broadband
spectral evolution of the source for different values of the inferred mass
accretion rate by studying energy spectra from selected regions in the Z-track
of its Color-Color Diagram. A two-component model, consisting of a soft thermal
component interpreted as thermal emission from an accretion disk and a thermal
Comptonization component, is unable to fit the whole 3--200 keV energy spectrum
at low accretion rates. Strong residuals in the highest energy band of the
spectrum require the addition of a third component that can be fitted with a
power-law component, that could represent a second thermal Comptonization from
a much hotter plasma, or a hybrid thermal/non-thermal Comptonization. We
discuss the physical implications derived from the results of our analysis,
with a particular emphasis on the hardest part of the X-ray emission and its
possible origins.Comment: 18 pages. Accepted for publication in Ap