5,498 research outputs found
Reflection at large distance from the central engine in Seyferts
We consider the possibility that most of the reflection component, observed
in the hard X-ray spectra of Seyfert galaxies, could be formed on an extended
medium, at large distance from the central source of primary radiation (e.g. on
a torus). Then, the reflector cannot respond to the rapid fluctuations of the
primary source. The observed reflected flux is controlled by the time-averaged
primary spectrum rather than the instantaneous (observed) one. We show that
this effect strongly influence the spectral fits parameters derived under the
assumption of a reflection component consistent with the primary radiation. We
find that a pivoting primary power-law spectrum with a nearly constant
Comptonised luminosity may account for the reported correlation between the
reflection amplitude and the spectral index .Comment: Proceeding of the meeting "X-ray emission from accretion onto black
hole" 20-23 June 2001, Hopkins University, Baltimore, USA (style file
jhuwkshp.sty included
X-ray polarimetric signatures induced by spectral variability in the framework of the receding torus model
Obscuring circumnuclear dust is a well-established constituent of active
galactic nuclei (AGN). Traditionally referred to as the receding dusty torus,
its inner radius and angular extension should depend on the photo-ionizing
luminosity of the central source. Using a Monte Carlo approach, we simulate the
radiative transfer between the multiple components of an AGN adopting model
constraints from the bright Seyfert galaxy NGC 4151. We compare our model
results to the observed near-IR to UV polarization of the source and predict
its X-ray polarization. We find that the 2-8 keV polarization fraction of a
standard AGN model varies from less then a few percent along polar viewing
angles up to tens of percent at equatorial inclinations. At viewing angles
around the type-1/type-2 transition the X-ray polarization variability differs
between a static or a receding torus scenario. In the former case, the expected
2-8 keV polarization of NGC 4151 is found to be 1.21% +/- 0.34% with a constant
polarization position angle, while in the later scenario it varies from 0.1% to
6% depending on the photon index of the primary radiation. Additionally, an
orthogonal rotation of the polarization position angle with photon energy
appears for very soft primary spectra. Future X-ray polarimetry missions will
be able to test if the receding model is valid for Seyfert galaxies seen at a
viewing angle close to the torus horizon. The overall stability of the
polarization position angle for photon indexes softer than {\Gamma} = 1.5
ensures that reliable measurements of X-ray polarization are possible. We
derive a long-term observational strategy for NGC 4151 assuming observations
with a small to medium-size X-ray polarimetry satellite.Comment: 10 pages, 8 Figures, accepted for publication in A&
The existence of warm and optically thick dissipative coronae above accretion disks
In the past years, several observations of AGN and X-ray binaries have
suggested the existence of a warm T around 0.5-1 keV and optically thick, \tau
~ 10-20, corona covering the inner parts of the accretion disk. These
properties are directly derived from spectral fitting in UV to soft-X-rays
using Comptonization models. However, whether such a medium can be both in
radiative and hydrostatic equilibrium with an accretion disk is still
uncertain. We investigate the properties of such warm, optically thick coronae
and put constraints on their existence. We solve the radiative transfer
equation for grey atmosphere analytically in a pure scattering medium,
including local dissipation as an additional heating term in the warm corona.
The temperature profile of the warm corona is calculated assuming it is cooled
by Compton scattering, with the underlying dissipative disk providing photons
to the corona. Our analytic calculations show that a dissipative thick,
(\tau_{cor} ~ 10-12) corona on the top of a standard accretion disk can reach
temperatures of the order of 0.5-1 keV in its upper layers provided that the
disk is passive. But, in absence of strong magnetic fields, the requirement of
a Compton cooled corona in hydrostatic equilibrium in the vertical direction
sets an upper limit on the Thomson optical depth \tau_{cor} < 5 . We show this
value cannot be exceeded independently of the accretion disk parameters.
However, magnetic pressure can extend this result to larger optical depths.
Namely, a dissipative corona might have an optical depth up to ~ 20 when the
magnetic pressure is 100 times higher that the gas pressure. The observation of
warm coronae with Thomson depth larger than ~ 5 puts tights constraints on the
physics of the accretion disk/corona systems and requires either strong
magnetic fields or vertical outflows to stabilize the system.Comment: 9 pages 6 figure, submitted to A&A, comments are welcom
A multi-flow model for microquasars
We present a new picture for the central regions of Black Hole X-ray
Binaries. In our view, these central regions have a multi-flow configuration
which consists in (1) an outer standard accretion disc down to a transition
radius r_J, (2) an inner magnetized accretion disc below r_J driving (3) a non
relativistic self-collimated electron-proton jet surrounding, when adequate
conditions for pair creation are met, (4) a ultra relativistic
electron-positron beam. This accretion-ejection paradigm provides a simple
explanation to the canonical spectral states, from radio to X/gamma-rays, by
varying the transition radius r_J and disc accretion rate independently. Large
values of r_J and low accretion rate correspond to Quiescent and Hard states.
These states are characterized by the presence of a steady electron-proton MHD
jet emitted by the disc below r_J. The hard X-ray component is expect to form
at the jet basis. When r_J becomes smaller than the marginally stable orbit
r_i, the whole disc resembles a standard accretion disc with no jet,
characteristic of the Soft state. Intermediate states correspond to situations
where r_J ~ r_i. At large accretion rate, an unsteady pair cascade process is
triggered within the jet axis, giving birth to flares and ejection of
relativistic pair blobs. This would correspond to the luminous intermediate
state, with its associated superluminal motions.Comment: 12 pages, 3 figures. Proceedings of ``High Energies in the
Highlands'', Fort-William, 27 June-1 July 200
The effects of a comptonizing corona on the appearance of the reflection components in accreting black hole spectra
We discuss the effects of a comptonizing corona on the appearance of the
reflection components, and in particular of the reflection hump, in the X-rays
spectra of accreting black holes. Indeed, in the framework of a thermal corona
model, we expect that part (or even all, depending on the coronal covering
factor) of the reflection features should cross the hot plasma, and thus suffer
Compton scattering, before being observed. We have studied in detail the
dependence of these effects on the physical (i.e. temperature and optical
depth) and geometrical (i.e. inclination angle) parameters of the corona,
concentrating on the slab geometry . Due to the smoothing and shifting towards
high energies of the comptonized reflection hump, the main effects on the
emerging spectra appear above 100 keV. We have also investigated the importance
of such effects on the interpretation of the results obtained with the standard
fitting procedures. We found that fitting Comptonization models, taking into
account comptonized reflection, by the usual cut-off power law + uncomptonized
reflection model, may lead to an underestimation of the reflection
normalization and an overestimation of the high energy cut-off. We discuss and
illustrate the importance of these effects by analysing recent observational
results as those of the galaxy NGC 4258. We also find that the comptonizing
corona can produce and/or emphasize correlations between the reflection
features characteristics (like the iron line equivalent width or the covering
fraction) and the X-ray spectral index similar to those recently reported in
the literature. We also underline the importance of these effects when dealing
with accurate spectral fitting of the X-ray background.Comment: 11 pages, 14 figures accepted for publication in MNRAS. Version
printable on US 8.5x11 pape
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