19 research outputs found
Simulations of X-ray spectral/timing properties in a propagation model of variability of accreting black holes
A phenomenological model of X-ray variability of accreting black holes is
considered, where the variable emission is attributed to multiple active
regions/perturbations moving radially towards the central black hole. The hard
X-rays are produced by inverse Compton upscattering of soft photons coming from
reprocessing/thermalization of the same hard X-rays. The heating rate of the
Comptonizing plasma is assumed to scale with the rate of dissipation of
gravitational energy while the supply of soft photons is assumed to diminish
towards the center. Two scenarios are considered: (1) an inner hot flow with
outer truncated standard accretion disc and (2) an accretion disc with an
active corona and a thick hot ionized skin. A variant of the model is also
considered, which is compatible with the currently discussed multi-Lorentzian
description of power spectral densities of X-ray lightcurves.
In the inner hot flow scenario the model can reproduce the observed Fourier
frequency resolved spectra observed in X-ray binaries, in particular the
properties of the reprocessed component as functions of Fourier frequency. In
the accretion disc with ionized skin scenario the reduction of soft photons due
to the ionized skin is insufficient to produce the observed characteristics.Comment: 8 pages, MNRAS, replaced with the accepted version, minor changes in
formulae in Sec. 3.
Modelling the variability of the Fe K alpha line in accreting black holes
The variability of the Fe K alpha line near 6.5 keV seems to be reduced
compared to the variability of the hard X-rays which presumably drive the line
emission. This is observed both in active galactic nuclei and galactic black
hole binaries. We point out that such reduced variability, as well as lack of
coherence between the variations of the line and the continuum, are a natural
prediction of a propagation model of variability in the geometry of inner hot
accretion flow. We compute detail model predictions of the variability
characteristics which could be compared with current and future data. We also
point out that the model requires a gradual disappearance of the cold disc,
rather than a sharp transition from the cold disc to a hot flow.Comment: 7 pages, MNRAS, accepte
On variability and spectral distortion of the fluorescent iron lines from black-hole accretion discs
We investigate properties of iron fluorescent line arising as a result of
illumination of a black hole accretion disc by an X-ray source located above
the disc surface. We study in details the light-bending model of variability of
the line, extending previous work on the subject.
We indicate bending of photon trajectories to the equatorial plane, which is
a distinct property of the Kerr metric, as the most feasible effect underlying
reduced variability of the line observed in several objects. A model involving
an X-ray source with a varying radial distance, located within a few central
gravitational radii around a rapidly rotating black hole, close to the disc
surface, may explain both the elongated red wing of the line profile and the
complex variability pattern observed in MCG--6-30-15 by XMM-Newton.
We point out also that illumination by radiation which returns to the disc
(following the previous reflection) contributes significantly to formation of
the line profile in some cases. As a result of this effect, the line profile
always has a pronounced blue peak (which is not observed in the deep minimum
state in MCG--6-30-15), unless the reflecting material is absent within the
innermost 2--3 gravitational radii.Comment: 24 pages, 22 figures. Accepted for publication in MNRA
Accretion Disk Spectra of the Ultra-luminous X-ray Sources in Nearby Spiral Galaxies and Galactic Superluminal Jet Sources
Ultra-luminous Compact X-ray Sources (ULXs) in nearby spiral galaxies and
Galactic superluminal jet sources share the common spectral characteristic that
they have unusually high disk temperatures which cannot be explained in the
framework of the standard optically thick accretion disk in the Schwarzschild
metric. On the other hand, the standard accretion disk around the Kerr black
hole might explain the observed high disk temperature, as the inner radius of
the Kerr disk gets smaller and the disk temperature can be consequently higher.
However, we point out that the observable Kerr disk spectra becomes
significantly harder than Schwarzschild disk spectra only when the disk is
highly inclined. This is because the emission from the innermost part of the
accretion disk is Doppler-boosted for an edge-on Kerr disk, while hardly seen
for a face-on disk. The Galactic superluminal jet sources are known to be
highly inclined systems, thus their energy spectra may be explained with the
standard Kerr disk with known black hole masses. For ULXs, on the other hand,
the standard Kerr disk model seems implausible, since it is highly unlikely
that their accretion disks are preferentially inclined, and, if edge-on Kerr
disk model is applied, the black hole mass becomes unreasonably large (> 300
M_solar). Instead, the slim disk (advection dominated optically thick disk)
model is likely to explain the observed super-Eddington luminosities, hard
energy spectra, and spectral variations of ULXs. We suggest that ULXs are
accreting black holes with a few tens of solar mass, which is not unexpected
from the standard stellar evolution scenario, and that their X-ray emission is
from the slim disk shining at super-Eddington luminosities.Comment: ApJ, accepte
Simultaneous Chandra and RXTE observations of the nearby bright Seyfert 2 galaxy NGC4945
We analyze recent simultaneous Chandra/RXTE observations of the Seyfert 2
galaxy NGC 4945. The unprecedented spatial resolution of Chandra means we are
able to separate the spectra of the nucleus, starburst and superwind regions,
while the RXTE data extend the spectrum to higher energies. The extreme
absorbing column of cm means that the nucleus
is only seen directly above 8--10 keV, while the lower energy spectrum from the
nuclear region in Chandra is dominated by reflection. By contrast, the
superwind is dominated by emission from hot plasma, but the starburst region
contains both hot plasma and reflection signatures. To form a reflected
spectrum requires that the starburst region contains clumps of cool, optically
thick material, perhaps star forming cores, which are irradiated by 7--10 keV
photons from the nucleus. Since photons of this energy are obscured along the
line sight then this confirms the result of Madejski et al. (2000) that the
extreme absorbtion material is disk-like rather than a torus. However, the
IR/optical limits on the lack of high excitation emission lines show that by
contrast the lower energy photons from the nucleus are obscured in all
directions. We discuss the complex absorption structure revealed by these
observations, and propose an an overall source geometry in which the nucleus is
completely embedded in material with cmComment: Accepted in Ap
On the influence of relativistic effects on X-ray variability of accreting black holes
ABSTRACT X-rays produced by compact flares corotating 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 we compute the expected strength of the relativistic signal in current data of Seyfert galaxies and black hole binaries. Such signals can clearly be seen in, for example, recent XMM-Newton data from MCG-6-30-15, if indeed the X-rays are produced by corotating flares concentrated toward the inner disc edge around an extreme Kerr black hole. The 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
Modelling the energy dependencies of X-ray quasi-periodic oscillations in accreting compact objects
ABSTRACT We have constructed models of quasi-periodic variability of X-ray emission from accreting compact objects. Assuming a general scenario of a propagation model of variability, with inverse Compton upscatering as the emission mechanism, we have considered a number of cases for the periodic modulation: modulation of the plasma heating rate, cooling rate by external soft photons, and the amplitude of the reprocessed component. We have computed various observational characteristics which can be compared to good quality data. These include Fourier-frequency resolved spectra and results of cross-correlation analysis between light-curves at different energies. Each model of modulation predicts specific observational signatures, which help in identifying the physical processes driving QPO emission in accreting sources