25 research outputs found
Predictions for Fourier-resolved X-ray spectroscopy from the model of magnetic flare avalanches above an accretion disc with hot ionized skin
The magnetic flare avalanches model of Poutanen & Fabian for X-ray
variability of accreting black holes is combined with computations of vertical
structure of illuminated accretion discs in hydrostatic equilibrium. The latter
predict the existence of a hot ionized skin, due to the thermal instability of
X-ray illuminated plasma. The presence of such ionized skin, with properties
dependent on disc radius, introduces a dependence of the emitted X-ray spectrum
on the position on the disc. If the position is related to the time scale of
the flares, the X-ray energy spectra (both the primary continuum and the
reprocessed component) gain an additional dependence on Fourier frequency,
beside that resulting from spectral evolution during a flare. We compute the
Fourier frequency resolved spectra in this model and demonstrate that the
presence of the hot skin introduces trends opposite to those observed in black
hole binaries. Furthermore, the flare profile is strongly constrained, if the
Fourier frequency dependence due to spectral evolution is to agree with
observations.Comment: 10 pages, MNRAS, in pres
Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the line emission. These lines serve as powerful probes for examining
the structure of inner regions of accretion disks. Previous studies of line
emission have considered geometrically thin disks only, where the gas moves
along geodesics in the equatorial plane of a black hole. Here we extend this
work to consider effects on line profiles from finite disk thickness, radial
accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution,
and find that within this framework, turbulent broadening is the dominant new
effect. The most prominent change in the skewed, double-horned line profiles is
a substantial reduction in the maximum flux at both red and blue peaks. The
effect is most pronounced when the inclination angle is large, and when the
accretion rate is high. Thus, the effects discussed here may be important for
future detailed modeling of high quality observational data.Comment: 21 pages including 8 figures; LaTeX; ApJ format; accepted by ApJ;
short results of this paper appeared before as a conference proceedings
(astro-ph/9711214
An extended scheme for fitting X-ray data with accretion disk spectra in the strong gravity regime
Accreting black holes are believed to emit X-rays which then mediate
information about strong gravity in the vicinity of the emission region. We
report on a set of new routines for the Xspec package for analysing X-ray
spectra of black-hole accretion disks. The new computational tool significantly
extends the capabilities of the currently available fitting procedures that
include the effects of strong gravity, and allows one to systematically explore
the constraints on more model parameters than previously possible (for example
black-hole angular momentum). Moreover, axial symmetry of the disk intrinsic
emissivity is not assumed, although it can be imposed to speed up the
computations. The new routines can be used also as a stand-alone and flexible
code with the capability of handling time-resolved spectra in the regime of
strong gravity. We have used the new code to analyse the mean X-ray spectrum
from the long XMM--Newton 2001 campaign of the Seyfert 1 galaxy MCG--6-30-15.
Consistent with previous findings, we obtained a good fit to the broad Fe K
line profile for a radial line intrinsic emissivity law in the disk which is
not a simple power law, and for near maximal value of black hole angular
momentum. However, equally good fits can be obtained also for small values of
the black hole angular momentum. The code has been developed with the aim of
allowing precise modelling of relativistic effects. Although we find that
current data cannot constrain the parameters of black-hole/accretion disk
system well, the approach allows, for a given source or situation, detailed
investigations of what features of the data future studies should be focused on
in order to achieve the goal of uniquely isolating the parameters of such
systems.Comment: Accepted for publication in ApJ S
The Nature of the Emission Components in the Quasar/NLS1 PG1211+143
We present the study of the emission properties of the quasar PG1211+143,
which belongs to the class of Narrow Line Seyfert 1 galaxies. On the basis of
observational data analyzed by us and collected from the literature, we study
the temporal and spectral variability of the source in the optical/UV/X-ray
bands and we propose a model that explains the spectrum emitted in this broad
energy range. In this model, the intrinsic emission originating in the warm
skin of the accretion disk is responsible for the spectral component that is
dominant in the softest X-ray range. The shape of reflected spectrum as well as
Fe K line detected in hard X-rays require the reflecting medium to be mildly
ionized (xi~500). We identify this reflector with the warm skin of the disk and
we show that the heating of the skin is consistent with the classical alpha
P_{tot} prescription, while alpha P_{gas} option is at least two orders of
magnitude too low to provide the required heating. We find that the mass of the
central black hole is relatively small (M_BH~10^7- 10^8 Msun, which is
consistent with the Broad Line Region mapping results and characteristic for
NLS1 class.Comment: 22 pages, 10 figures, accepted to Ap
A Structure for Quasars
This paper proposes a simple, empirically derived, unifying structure for the
inner regions of quasars. This structure is constructed to explain the broad
absorption line (BAL) regions, the narrow `associated' ultraviolet and X-ray
warm absorbers (NALs); and is also found to explain the broad emission line
regions (BELR), and several scattering features, including a substantial
fraction of the broad X-ray Iron-K emission line, and the bi-conical extended
narrow emission line region (ENLR) structures seen on large kiloparsec scales
in Seyfert images. Small extensions of the model to allow luminosity dependent
changes in the structure may explain the UV and X-ray Baldwin effects and the
greater prevalence of obscuration in low luminosity AGN.Comment: 35 pages, including 8 color figures (figures 4abc are big).
Astrophysical Journal, in press. Expanded version of conference paper
astro-ph/000516
An optically thick inner corona geometry for the Very High State Galactic Black Hole XTE J1550-564
(truncated version) The X-ray spectra of Galactic binary systems in the very
high state show both strong disk emission and a strong, steep tail to high
energies. We use simultaneous optical-ASCA-RXTE data from the black hole
transient XTE J1550-564 as a specific example, and show that these have disc
spectra which are significantly lower in temperature than those seen from the
same source at the same luminosity when in the disc dominated state. If these
give a true picture of the disc then either the disc emissivity has reduced,
and/or the disc truncates above the last stable orbit. However, it is often
assumed that the tail is produced by Compton scattering, in which case its
shape in these spectra requires that the Comptonising region is marginally
optically thick (tau~2-3), and covers a large fraction of the inner disc. This
will distort our view of the disc. We build a theoretical model of a
Comptonising corona over an inner disc, and fit this to the data, but find that
it still requires a large increase in inner disc radius for a standard disc
emissivity. Instead it seems more probable that the disc emissivity changes in
the presence of the corona. We implement the specific inner disc-corona
coupling model of Svensson & Zdziarski (1994) and show that this can explain
the low temperature/high luminosity disc emission seen in the very high state
with only a small increase in radius of the disc. While this inferred disc
truncation is probably not significant given the model uncertainties, it is
consistent with the low frequency QPO and gives continuity of properties with
the low/hard state spectra.Comment: MNRAS accepted version, with major expansion of the discussion to
include comments on comptonisation, extreme broad iron lines and high
frequency QPO's. 18 pages, 8 figure
On the Origin of Broad Fe K alpha and Hi H alpha Lines in AGN
We examine the properties of the Fe emission lines that arise near 6.4 keV in
the ASCA spectra of AGN. Our emphasis is on the Seyfert 1 galaxies where broad
and apparently complex Fe K alpha emission is observed. We consider various
origins for the line but focus on the pros and cons for line emitting accretion
disk models. We develop a simple model of an illuminated disk capable of
producing both X-ray and optical lines from a disk. The model is able to
reproduce the observed Fe K alpha FWHM ratio as well as the radii of maximum
emissivity implied by the profile redshifts. The overall profile shapes however
do not fit well the predictions of our disk illumination model nor do we derive
always consistent disk inclinations for the two lines. We conclude that the
evidence for and against an accretion disk origin for the Fe K alpha emission
is equal at best. The bulk of the data requires a very disparate set of line
fits which shed little light on a coherent physical model. We briefly consider
alternatives to disk emission models and show that a simple bicone model can
reproduce the FE line profiles equally well.Comment: 29 pages, 6 tables, 6 figures. Submitted for publication in the
Astrophysical Journal part
The effects of relativistic bulk motion of X-ray flares in the corona on the iron Kalpha line in Seyfert 1 galaxies
We study the effects of the bulk motion of X-ray flares on the shape and
equivalent width of the iron Kalpha line from an untruncated cold disk around a
Kerr black hole using fully relativistic calculations. The flares are located
above a cold accretion disk -- either on or off the rotation axis. For on- or
off-axis flares, the upward/outward bulk motion causes a reduction of the iron
Kalpha line width. To a distant observer with a low inclination angle (\theta_o
\simlt 30deg.), larger upward/outward bulk velocities decrease the extension of
the red wing, with little change in the location of the blue `edge'. In
contrast, an observer at a large inclination angle (e.g. \theta_o=60deg.) sees
both the red wing and the blue `edge' change with the bulk velocity. The
equivalent width of the iron Kalpha line decreases rapidly with increasing bulk
velocity of flares. However, the `narrower' line profiles observed in some
objects (e.g. IC4329A and NGC4593) are difficult to produce using the
out-flowing magnetic flare model with an appropriate equivalent width unless
the X-ray emission is concentrated in an outer region with a radius of several
tens of r_g=GM/c^2 or more. An important result is that the iron Kalpha line
intensity is found to be constant even though the continuum flux varies
significantly, which is true for out-flowing magnetic flares with different
bulk velocities but similar intrinsic luminosities when located close to the
central black hole. We find that fluctuations in the bulk velocities of
out-flowing low-height flares located at the inner region (r\simlt 15r_g) can
account for a constant iron Kalpha line and significant continuum variation as
observered in MCG-6-30-15 and NGC5548. (Abridged)Comment: 30 pages (including 8 figures); minor changes, to appear in ApJ, Nov.
10, 200
The X-ray spectrum of the Seyfert I galaxy Mrk 766: Dusty Warm Absorber or Relativistic Emission Lines?
Competing models for broad spectral features in the soft X-ray spectrum of
the Seyfert I galaxy Mrk766 are tested against data from a 130 ks XMM-Newton
observation. A model including relativistically broadened Ly-alpha emission
lines of OVIII, NVII and CVI is a better fit to 0.3-2 keV XMM RGS data than a
dusty warm absorber. Moreover, the measured depth of neutral iron absorption
lines in the spectrum is inconsistent with the magnitude of the iron edge
required to produce the continuum break at 17-18Angstroms in the dusty warm
absorber model. The relativistic emission line model can reproduce the
broad-band (0.1-12 keV) XMM-EPIC data with the addition of a fourth line to
represent emission from ionized iron at 6.7 keV and an excess due to reflection
at energies above the iron line. The profile of the 6.7 keV iron line is
consistent with that measured for the low energy lines. There is evidence in
the RGS data at the 3sigma level for spectral features that vary with source
flux. The covering fraction of warm absorber gas is estimated to be ~12%. Iron
in the warm absorber is found to be overabundant with respect to CNO compared
to solar values.Comment: Accepted for publication in ApJ Letter
Estimation of relativistic accretion disk parameters from iron line emission
The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide
strong evidence for an accretion disk near a supermassive black hole as a
source of the emission. Here we present an analysis of the geometrical and
kinematic properties of the disk based on the extreme frequency shifts of a
line profile as determined by measurable flux in both the red and blue wings.
The edges of the line are insensitive to the distribution of the X-ray flux
over the disk, and hence provide a robust alternative to profile fitting of
disk parameters. Our approach yields new, strong bounds on the inclination
angle of the disk and the location of the emitting region. We apply our method
to interpret observational data from MCG-6-30-15 and find that the commonly
assumed inclination 30 deg for the accretion disk in MCG-6-30-15 is
inconsistent with the position of the blue edge of the line at a 3 sigma level.
A thick turbulent disk model or the presence of highly ionized iron may
reconcile the bounds on inclination from the line edges with the full line
profile fits based on simple, geometrically thin disk models. The bounds on the
innermost radius of disk emission indicate that the black hole in MCG-6-30-15
is rotating faster than 30 % of theoretical maximum. When applied to data from
NGC 4151, our method gives bounds on the inclination angle of the X-ray
emitting inner disk of 50\pm 10 deg, consistent with the presence of an
ionization cone grazing the disk as proposed by Pedlar et al. (1993). The
frequency extrema analysis also provides limits to the innermost disk radius in
another Seyfert-1 galaxy, NGC 3516.Comment: 45 pages including 20 figures; LaTeX; ApJ format; accepted by ApJ;
for higher resolution figures see
http://pegasus.as.arizona.edu/~vpariev/disk_paper2.ps.g