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

A quasi-spherical inner accretion flow in Seyfert galaxies ?

We study a phenomenological model for the continuum emission of Seyfert galaxies. In this quasi-spherical accretion scenario, the central X-ray source is constituted by a hot spherical plasma region surrounded by spherically distributed cold dense clouds. The cold material is radiatively coupled with the hot thermal plasma. Assuming energy balance, we compute the hard X-ray spectral slope Gamma and reflection amplitude R. This simple model enables to reproduce both the range of observed hard X-ray spectral slopes, and reflection amplitude R. It also predicts a correlation between R and Gamma which is very close to what is observed. Most of the observed spectral variations from source to source, would be due to differences in the cloud covering fraction. If some internal dissipation process is active in the cold clouds, darkening effects may provide a simple explanation for the observed distributions of reflection amplitudes, spectral slopes, and UV to X-ray flux ratios.Comment: 12 pages, 9 figures, accepted for publication in MNRA

Accretion disc coronae in black hole binaries

Most of the luminosity of accreting black hole is emitted in the X-ray band. This radiation is believed to emerge, through inverse Compton process, from a hot (Te ~ 10^8 -10^9 K) optically thin (Thomson optical depth ~ 1) plasma probably located in the immediate vicinity of the black hole. The mechanisms at work in this so called Compton corona can be unveiled through hard X-ray observations which have revealed a rich phenomenology. Depending on luminosity different spectral states are observed suggesting that the nature and geometry of the corona depends on mass accretion rate. In many instances the spectral behaviour as a function of luminosity shows some degree of hysteresis. The mechanisms triggering the transition between spectral states is very unclear although it could be related to an evaporation/condensation equilibrium in an accretion disc corona system. From the observation of correlation between the X-ray and radio band, it appears that the Compton corona is intimately related to the formation of compact jets and probably constitutes the base of the jet.Comment: 8 pages, 1 figure; invited review at the 'Coronae of Stars and Accretion Disks' conference, Bonn, 12-13 December 200

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

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