Prague Relativistic Astrophysics

Nick Kylafis

Black-hole X-ray transients (BHTs) often exhibit during their outbursts a characteristic q-shaped curve in a hardness-luminosity diagram. A rich phenomenology has been accumulated over the years regarding this diagram. It is desirable to have a physical picture of BHTs over the entire q-shaped curve, which hopefully will have predictive power. Such a physical picture is proposed here and it relies on 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 rates. The mass-accretion rate increases, reaches values close to the Eddington rate, and decreases again. The second assumption is that at low accretion rates the accretion disk is geometrically thick, ADAF-like, while at high accretion rates it is thin. This assumption is generally accepted. Unlike phenomenological pictures typically invoked for BHTs, our physical picture explains a) the difference between type C and type B QPOs, b) the formation and the destruction of jets, and c) why BHTs traverse the q-shaped curve always in the counterclockwise direction and that no BHT is expected to ever traverse the entire q-curve in the clockwise direction. Our physical picture explains the q-shaped curve and its associated phenomenology with only one parameter, the accretion rate.