Accretion disc coronae in black hole binaries

TL;DR

This paper reviews the properties and behaviors of accretion disc coronae in black hole binaries, emphasizing their spectral states, transition mechanisms, and connection to jet formation based on X-ray observations.

Contribution

It synthesizes current understanding of corona physics, spectral state transitions, and the corona-jet connection in black hole binary systems.

Findings

Coronal properties vary with luminosity and spectral state.

Hysteresis observed in spectral state transitions.

Coronal activity is linked to jet formation.

Abstract

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.