Accretion and ejection in black-hole X-ray transients

Understand

Aims: We summarize the current observational picture of the outbursts of black-hole X-ray transients (BHTs), based on the evolution traced in a hardness-luminosity diagram (HLD), and we offer a physical interpretation.

• Methods: The basic ingredient in our interpretation is the Poynting-Robertson Cosmic Battery (PRCB, Contopoulos & Kazanas 1998), which provides locally the poloidal magnetic field needed for the ejection of the jet.
• In addition, we make 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.