293 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
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
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
Cyclotron-Synchrotron: harmonic fitting functions in the non-relativistic and trans-relativistic regimes
The present work investigates the calculation of absorption and emission
cyclotron line profiles in the non-relativistic and trans-relativistic regimes.
We provide fits for the ten first harmonics with synthetic functions down to
10^(-4) of the maximum flux with an accuracy of 20 per cent at worst. The lines
at a given particle energy are calculated from the integration of the Schott
formula over the photon and the particle solid angles relative to the magnetic
field direction. The method can easily be extended to a larger number of
harmonics. We also derive spectral fits of thermal emission line plasmas at
non-relativistic and trans-relativistic temperatures extending previous
parameterisations.Comment: 11 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
O gálio e a patologia óssea.
Estima-se que mais de 200.000 cirurgias para fusĂŁo da coluna vertebral sĂŁo realizadas a cada ano nos EUA. Artrodese lombar posterolateral intertransversalis Ă© o procedimento mais comum realizado, ainda que a falha para obter uma sĂłlida uniĂŁo Ăłssea ocorre em 10% a 40% dos pacientes com Ăşnico nĂvel envolvido e mais freqĂĽentemente em mĂşltiplos nĂveis. Esta alta taxa de pseudoartrose indica que eventos fisiolĂłgicos, biolĂłgicos e quĂmicos, cruciais para este processo, nĂŁo sĂŁo adequadamente compreendidos. A nĂŁo uniĂŁo Ăłssea frequentemente leva a uma resolução insatisfatĂłria dos sintomas clĂnicos e usualmente resulta em maior custo mĂ©dico e morbidade, bem como a necessidade de intervenções adicionais(1). Infelizmente, o efeito dos Ăons metálicos no processo de mineralização nĂŁo tem recebido considerável atenção atĂ© recentemente, entretanto dados interessantes da participação do alumĂnio e gálio no metabolismo Ăłsseo foram publicados há mais de 15 anos(2). Atualmente, o nĂşmero de publicações dedicadas ao papel do gálio na patologia Ăłssea está crescendo rapidamente, todavia, revisões abrangentes nĂŁo estĂŁo disponĂveis. A intenção do presente estudo Ă© preencher esta lacuna, considerando a formação, crescimento e solubilidade da hidroxiapatita na presença de sais de gálio, a incorporação do gálio dentro do tecido Ăłsseo e o mecanismo de atividade terapĂŞutica deste elemento
On the X-ray spectra of luminous, inhomogeneous accretion flows
We discuss the expected X-ray spectral and variability properties of black
hole accretion discs at high luminosity, under the hypothesis that radiation
pressure dominated discs are subject to violent clumping instabilities and, as
a result, have a highly inhomogeneous two-phase structure. After deriving the
full accretion disc solutions explicitly in terms of the parameters of the
model, we study their radiative properties both with a simple two-zones model,
treatable analytically, and with radiative transfer simulations which account
simultaneously for energy balance and Comptonisation in the hot phase, together
with reflection, reprocessing, ionization and thermal balance in the cold
phase. We show that, if not only the density, but also the heating rate within
these flows is inhomogeneous, then complex reflection-dominated spectra can be
obtained for a high enough covering fraction of the cold phase. In general,
large reflection components in the observed X-ray spectra should be associated
with strong soft excesses, resulting from the combined emission of ionized
atomic emission lines. The variability properties of such systems are such
that, even when contributing to a large fraction of the hard X-ray spectrum,
the reflection component is less variable than the power-law like emission
originating from the hot Comptonising phase, in agreement with what is observed
in many Narrow Line Seyfert 1 galaxies and bright Seyfert 1. Our model falls
within the family of those trying to explain the complex X-ray spectra of
bright AGN with ionized reflection, but presents an alternative, specific,
physically motivated, geometrical setup for the complex multi-phase structure
of the inner regions of near-Eddington accretion flows.Comment: 15 pages, 9 figures. MNRAS, in pres
Properties of AGN coronae in the NuSTAR era – II. Hybrid plasma
The corona, a hot cloud of electrons close to the centre of the accretion disc, produces the hard X-ray power-law continuum commonly seen in luminous active galactic nuclei. The continuum has a high-energy turnover, typically in the range of one to several 100 keV and is suggestive of Comptonization by thermal electrons. We are studying hard X-ray spectra of AGN obtained with NuSTAR after correction for X-ray reflection and under the assumption that coronae are compact, being only a few gravitational radii in size as indicated by reflection and reverberation modelling. Compact coronae raise the possibility that the temperature is limited and indeed controlled by electron–positron pair production, as explored earlier (Paper I). Here, we examine hybrid plasmas in which a mixture of thermal and non-thermal particles is present. Pair production from the non-thermal component reduces the temperature leading to a wider temperature range more consistent with observations
Jet disc coupling in black hole binaries
In the last decade multi-wavelength observations have demonstrated the
importance of jets in the energy output of accreting black hole binaries. The
observed correlations between the presence of a jet and the state of the
accretion flow provide important information on the coupling between accretion
and ejection processes. After a brief review of the properties of black hole
binaries, I illustrate the connection between accretion and ejection through
two particularly interesting examples. First, an INTEGRAL observation of Cygnus
X-1 during a 'mini-' state transition reveals disc jet coupling on time scales
of orders of hours. Second, the black hole XTEJ1118+480 shows complex
correlations between the X-ray and optical emission. Those correlations are
interpreted in terms of coupling between disc and jet on time scales of seconds
or less. Those observations are discussed in the framework of current models.Comment: Invited talk at the Fifth Stromlo Symposium: Disks, Winds & Jets -
from Planets to Quasars. Accepted for publication in Astrophysics & Space
Scienc
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