A unified accretion-ejection paradigm for black hole X-ray binaries. III. Spectral signatures of hybrid disk configurations

Context. It has been suggested that the cycles of activity of X-ray binaries (XRB) are triggered by a switch in the dominant disk torque responsible for accretion. As the disk accretion rate increases, the disk innermost regions therefore change from a jet-emitting disk (JED) to a standard accretion disk (SAD).
Aims: While JEDs have been proven to successfully reproduce X-ray binary hard states, the existence of an outer cold SAD introduces an extra nonlocal cooling term. We investigate the thermal structure and associated spectra of such a hybrid disk configuration.
Methods: We use a two-temperature plasma code, allowing for outside-in computation of the disk local thermal equilibrium with self-consistent advection and optically thin-to-thick transitions in both radiation and gas supported regimes. The nonlocal inverse Compton cooling introduced by the external soft photons is computed by the BELM code.
Results: This additional cooling term has a profound influence on JED solutions, allowing a smooth temperature transition from the outer SAD to the inner JED. We explore the full parameter space in disk accretion rate and transition radius, and show that the whole domain in X-ray luminosities and hardness ratios covered by standard XRB cycles is well reproduced by such hybrid disk configurations. Precisely, a reasonable combination of these parameters allows us to reproduce the 3-200 keV spectra of each of five canonical XRB states. Along with these X-ray signatures, JED-SAD configurations also naturally account for the radio emission whenever it is observed.
Conclusions: By varying only the radial transition radius and the accretion rate, hybrid disk configurations combining an inner JED and an outer SAD are able to simultaneously reproduce the X-ray spectral states and radio emission of X-ray binaries during their outburst. Adjusting these two parameters, it is then possible to reproduce a full cycle. This will be shown in a forthcoming paper.