Magnetic flares in accretion disc coronae and the spectral states of blackhole candidates: the case of GX339-4

We examine the constraints that the observations of different spectral stat es displayed by Galactic black hole candidates impose on the properties of magnetic flares resulting from the reconnection of flux tubes that rise fro m the accretion disc into a corona because of the magnetic buoyancy (Parker ) instability. Using observations of one of the best-studied examples, GX33 9-4, we identify the geometry and physical conditions characterizing each o f these states. We find that, if in the soft state flaring occurs at small scaleheights above the accretion disc, a soft thermal-like spectrum, charac teristic of this state, can result from the heating and consequent reradiat ion of the hard X-rays produced by such flares. The hard tail can then be p roduced by Comptonization of the soft radiation. Conversely, the hard state may result from a phase in which flares are triggered high above the under lying accretion disc and produce X-rays via Comptonization of either intern al cyclo-synchrotron radiation or soft disc photons. The spectral character istics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disc the system is phot on-starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disc greatly enhances the local soft-photon field with the result that the spectrum softens. We interpret these two states as bei ng related to two different phases of magnetic energy dissipation. We specu late that, in the soft state, Parker instability in the disc may favour the emergence of large numbers of relatively low-magnetic-field flux tubes. In the hard state, only intense magnetic fields become buoyant and magnetic l oops are able to rise and expand in the coronal atmosphere. This possibilit y can also qualitatively account for the observed short time-scale variabil ity and the characteristics of the X-ray-reflected component of the hard st ate.