Models for microquasars

I review current models used to interpret the spectra and variability of microquasars. Among other things, I discuss the structure of the accretion flow and its dependence on mass accretion rate, the intrinsic connection between hot comptonizing corona and compact radio jet in the hard state, as well as possible models for the spectral hysteresis observed during outbursts of transient sources. Finally I comment on several models for the non-poissonian X-ray noise in black hole binaries which, at least in some instances, is suspected to be associated with some form of coupling between disc and jet activity.Comment: 15 pages, 3 figures, invited review at the Sixth Microquasar workshop: Microquasars and Beyond, Como, Italy, September 18-22, 200

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 $R$ and the spectral index $\Gamma$.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

Jet-Disc coupling in the accreting black hole XTEJ1118+480

We interpret the rapid correlated UV/optical/ X-ray variability of XTEJ1118+480 as a signature of the coupling between the X-ray corona and a jet emitting synchrotron radiation in the optical band.We propose a scenario in which the jet and the X-ray corona are fed by the same energy reservoir where large amounts of accretion power are stored before being channelled into either the jet or the high energy radiation. This time dependent model reproduces the main features of the rapid multi-wavelength variability of XTEJ1118+480. A strong requirement of the model is that the total jet power should be at least a few times larger than the observed X-ray luminosity. This would be consistent with the overall low radiative efficiency of the source. We present independent arguments showing that the jet probably dominates the energetic output of all accreting black holes in the low-hard state.Comment: 8 pages, 1 figure, to appear in the proceedings of "From X-ray binaries to quasars: Black hole accretion on all mass scales, (Amsterdam, July 2004)", Eds. T. Maccarone, R. Fender, L. H

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