2,659 research outputs found
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
Energy Loss and Flavor Dynamics from Single Particle Measurements in PHENIX
The transverse momentum spectra, yields, and ratios of charged pions,
protons, and antiprotons have been studied up to 5 GeV/c in in 5
different centrality classes in Au+Au collisions at = 200 GeV.
These results are compared and contrasted with the observables calculated in
recombination models of hadronization. They are also used to examine the color
charge dependence of parton energy loss in the medium.Comment: 4 pages, 6 figures, to appear in the Proceedings of Quark Matter 200
Evaluation of meterological rocket data
Meteorological rocket data compared with rawinsonde observation
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
Numerical computation of isotropic Compton scattering
Compton scattering is involved in many astrophysical situations. It is well
known and has been studied in detail for the past fifty years. Exact formulae
for the different cross sections are often complex, and essentially asymptotic
expressions have been used in the past. Numerical capabilities have now
developed to a point where they enable the direct use of exact formulae in
sophisticated codes that deal with all kinds of interactions in plasmas.
Although the numerical computation of the Compton cross section is simple in
principle, its practical evaluation is often prone to accuracy issues. These
can be severe in some astrophysical situations but are often not addressed
properly. In this paper we investigate numerical issues related to the
computation of the Compton scattering contribution to the time evolution of
interacting photon and particle populations. An exact form of the isotropic
Compton cross section free of numerical cancellations is derived. Its accuracy
is investigated and compared to other formulae. Then, several methods to solve
the kinetic equations using this cross section are studied. The regimes where
existing cross sections can be evaluated numerically are given. We find that
the cross section derived here allows for accurate and fast numerical
evaluation for any photon and electron energy. The most efficient way to solve
the kinetic equations is a method combining a direct integration of the cross
section over the photon and particle distributions and a Fokker-Planck
approximation. Expressions describing this combination are given.Comment: 11 pages. Accepted for publication in A&
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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