Accretion and Ejection in Black-Hole X-Ray Transients

A rich phenomenology has been accumulated over the years regarding accretion and ejection in black-hole X-ray transients (BHTs). Here we summarize the current observational picture of the outbursts of BHTs, based on the evolution traced in a hardness – luminosity diagram (HLD), and we offer a physical interpretation with two assumptions, easily justifiable. The first is that the mass-accretion rate to the black hole in a BHT outburst has a generic bell-shaped form. This is guaranteed by the observational fact that all BHTs start their outburst and end it at the quiescent state, i.e., at very low accretion rate. The second assumption is that at low accretion rates the accretion flow is geometrically thick, ADAF-like, while at high accretion rates it is geometrically thin. Both, at the beginning and the end of an outburst, a strong poloidal magnetic field develops locally in the ADAF-like part of the accretion flow, and this explains naturally why a jet is always present in the right part of the HLD. “Memory” of the system explains naturally why BHTs traverse the q-shaped curves in the HLD always in the counterclockwise direction and that no BHT is expected to ever traverse the entire curve in the clockwise direction. The only parameter in our picture is the accretion rate.