362 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
Radiation processes around accreting black holes
Accreting sources such as AGN, X-ray binaries or gamma-ray bursts are known
to be strong, high energy emitters. The hard emission is though to originate
from plasmas of thermal and/or non-thermal high energy particles. Not only does
this emission allow to probe the unique properties of the matter in an extreme
environment, but it also has a crucial backreaction on the energetics and the
dynamics of the emitting medium itself. Understanding interactions between
radiation and matter has become a key issue in the modelling of high energy
sources. Although most cross sections are well known, they are quite complex
and the way all processes couple non-linearly is still an open issue.
We present a new code that solves the local, kinetic evolution equations for
distributions of electrons, positrons and photons, interacting by radiation
processes such as self-absorbed synchrotron and brems-strahlung radiation,
Compton scattering, pair production/annihilation, and by Coulomb collisions.
The code is very general and aimed to modelled various high energy sources. As
an application, we study the spectral states of X-ray binaries, including
thermalization by Coulomb collisions and synchrotron self-absorption. It is
found that the low-hard and high-soft states can be modelled with different
illumination but the same non-thermal acceleration mechanism.Comment: 4 pages, 2 figures, proceedings of the SF2A conference 200
The x-ray corona and jet of cygnus x-1
Evidence is presented indicating that in the hard state of Cygnus X-1, the
coronal mag- netic field might be below equipartition with radiation
(suggesting that the corona is not powered by magnetic field dissipation) and
that the ion temperature in the corona is significantly lower than what
predicted by ADAF like models. It is also shown that the current estimates of
the jet power set interesting contraints on the jet velocity (which is at least
mildly relativistic), the accretion efficiency (which is large in both spectral
states), and the nature of the X-ray emitting region (which is unlikely to be
the jet).Comment: 8 pages, 1 figure. Accepted for publication in Journal of Modern
Physics D, Proceedings of HEPRO II conference, Buenos Aires, Argentina,
October 26-30, 200
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
The emission of Cygnus X-1: observations with INTEGRAL SPI from 20 keV to 2 MeV
We report on Cyg X-1 observations performed by the SPI telescope onboard the
INTEGRAL mission and distributed over more than 6 years. We investigate the
variability of the intensity and spectral shape of this peculiar source in the
hard X-rays domain, and more particularly up to the MeV region. We first study
the total averaged spectrum which presents the best signal to noise ratio (4 Ms
of data). Then, we refine our results by building mean spectra by periods and
gathering those of similar hardness.
Several spectral shapes are observed with important changes in the curvature
between 20 and 200 keV, even at the same luminosity level. In all cases, the
emission decreases sharply above 700 keV, with flux values above 1 MeV (or
upper limits) well below the recently reported polarised flux (Laurent et al.
2011), while compatible with the MeV emission detected some years ago by
CGRO/COMPTEL (McConnell et al., 2002).
Finally, we take advantage of the spectroscopic capability of the instrument
to seek for spectral features in the 500 keV region with negative results for
any significant annihilation emission on 2 ks and days timescales, as well as
in the total dataset.Comment: 14 pages, 10 figures, accepted for publication in Ap
Simulating acceleration and radiation processes in X-ray binaries
The high energy emission of microquasars is thought to originate from high
energy particles. Depending on the spectral state, the distribution of these
particles can be thermal with a high temperature (typically 100 keV) or
non-thermal and extending to even higher energy. The properties of high energy
plasmas are governed by a rich microphysics involving particle-particle
collisions and particles-photons interactions.
We present a new code developed to address the evolution of relativistic
plasmas. This one-zone code focuses on the microphysics and solves the coupled
kinetic equations for particles and photons, including Compton scattering,
synchrotron emission and absorption, pair production and annihilation,
bremsstrahlung emission and absorption, Coulomb interactions, and prescriptions
for additional particle acceleration and heating. It can in particular describe
mechanisms such a thermalisation by synchrotron self-absorption and Coulomb
collisions.
Using the code, we investigate whether various acceleration processes, namely
thermal heating, non-thermal acceleration and stochastic acceleration, can
reproduce the different spectral states of microquasars. Premilinary results
are presented.Comment: 9 pages, 6 figures, proceedings of the VII Microquasar Workshop:
Microquasars and Beyond, September 1-5 2008, Foca, Izmir, Turkey; accepted
for publication in Po
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