A unified paradigm for the spectral and temporal evolution of Black Hole X-ray Binaries

Abstract

We present a unified picture to explain the spectral states of BH binaries based on the two-flow model developed in our team since several years and already applied with success to AGNs. In our view, the central regions have a multi-flow configuration consisting in (1) an outer standard accretion disc (SAD) down to a transition radius rtr , (2) an inner magnetized jet emitting disc (JED) below rtr driving (3) a self-collimated non relativistic MHD jet surrounding, when conditions for pair creation are met, (4) a ultra relativistic pair beam. Large values of rtr correspond to Hard states while small values correspond to Soft states. In between these extremes, in the high intermediate state, rtr can reach values that switch on a pair cascade process giving birth to ultra-relativistic pair blobs that explain the superluminal events. In this model the accretion rate but also the disc magnetization μ play important roles, the latter being necessarily in the range 0.1–1 for the MHD jet to exist. Then, with simple assumptions on μ , we propose an explanation for the hysteresis behavior observed in microquasars during their outburst . We also discuss the nature, in our framework, of the X-ray corona. While it is commonly believed to be at the base of the jet it cannot be in the jet itself, its optical depth being to small to be consistent with the X-ray spectra observed in the Hard state. We show that it is expected to form naturally in the JED itself. Finally we investigate the nature of the X-ray variability in the hard state by assuming an oscillating hot plasma whose temperature and density vary locally through the propagation of a magneto-sonic wave. The variable comptonized spectra are computed through Monte-Carlo simulation. Preliminary results are discussed.