Towards a unified model for black hole X-ray binary jets

We present a unified semiquantitative model for the disc–jet coupling in black hole X-ray binary systems. In the process we have compiled observational aspects from the existing literature, as well as performing new analyses. We argue that during the rising phase of a black hole transient outburst the steady jet known to be associated with the canonical ‘low/hard’ state persists while the X-ray spectrum initially softens. Subsequently, the jet becomes unstable and an optically thin radio outburst is always associated with the soft X-ray peak at the end of this phase of softening. This peak corresponds to a ‘soft very high state’ or ‘steep power-law’ state. Softer X-ray states are not associated with ‘core’ radio emission. We further demonstrate quantitatively that the transient jets associated with these optically thin events are considerably more relativistic than those in the ‘low/hard’ X-ray state. This in turn implies that, as the disc makes its collapse inwards, the jet Lorentz factor rapidly increases, resulting in an internal shock in the outflow, which is the cause of the observed optically thin radio emission. We provide simple estimates for the efficiency of such a shock in the collision of a fast jet with a previously generated outflow that is only mildly relativistic. In addition, we estimate the jet power for a number of such transient events as a function of X-ray luminosity, and find them to be comparable to an extrapolation of the functions estimated for the ‘low/hard’ state jets. The normalization may be larger, however, which may suggest a contribution from some other power source such as black hole spin, for the transient jets. Finally, we attempt to fit these results together into a coherent semiquantitative model for the disc–jet coupling in all black hole X-ray binary systems