Magnetic flares in accretion disc coronae and the Spectral States of black hole candidates: the case of GX 339-4

We present a model for the different X-ray spectral states displayed by Galactic Black Hole Candidates (GBHC). We discuss the physical and spectral implications for a magnetically structured corona in which magnetic flares result from reconnection of flux tubes rising from the accretion disk by the magnetic buoyancy (Parker) instability. Using the observations of one of the best studied examples, GX339-4, we identify the geometry and the physical conditions characterizing each of these states. We find that, in the Soft state, flaring occurs at small scale heights above the accretion disk. The soft thermal–like spectrum, characteristic of this state, is the result of heating and consequent re-radiation of the hard X-rays produced by such flares. The hard tail is produced by Comptonization of the soft field radiation. Conversely, the hard state is the result of a phase in which flares are triggered high above the underlying accretion disk and produce X-rays via Comptonization of either internal cyclo–synchrotron radiation or of soft disk photons. The spectral characteristics of the different states are naturally accounted for by the choice of geometry: when flares are triggered high above the disk the system is photon–starved, hence the hard Comptonized spectrum of the hard state. Intense flaring close to the disk greatly enhanced the soft–photon field with the result that the spectrum softens. We interpret the two states as being related to two different phases of magnetic energy dissipation. In the Soft state, Parker instability in the disk favours the emergence of large numbers of relatively low magnetic field flux tubes. In the hard state, only intense magnetic fields become buoyant and magnetic loops are able to rise and expand in the coronal atmosphere. The model can also qualitatively account for the observed short timescale variability and the characteristics of the X-ray reflected component of the hard state.