A unified accretion-ejection paradigm for black hole X-ray binaries

Abstract

The spectral evolution of transient X-ray binaries can be reproduced by an interplay between two flows separated at a transition radius RJ: a standard accretion disk (SAD) in the outer parts beyond RJ and a jet-emitting disk (JED) in the inner parts. In the previous papers in this series we successfully recover the spectral evolution in both X-rays and radio for four outbursts of GX 339-4 by playing independently with the two parameters: RJ and the disk accretion rate Ṁin. In this paper we compare the temporal evolution of both RJ and Ṁin for the four outbursts. We show that despite the undeniable differences between the time evolution of each outburst, a unique pattern in the Ṁin−RJ plane seems to be followed by all cycles within the JED-SAD model. We call this pattern a fingerprint, and show that even the “failed” outburst considered follows it. We also compute the radiative efficiency in X-rays during the cycles and consider its impact on the radio–X-ray correlation. Within the JED-SAD paradigm, we find that the accretion flow is always radiatively efficient in the hard states, with between 15% and 40% of the accretion power being radiated away at any given time. Moreover, we show that the radiative efficiency evolves with the accretion rate because of key changes in the JED thermal structure. These changes give birth to two different regimes with different radiative efficiencies: the thick disk and the slim disk. While the existence of these two regimes is intrinsically linked to the JED-SAD model, we show direct observational evidence of the presence of two different regimes using the evolution of the X-ray power-law spectral index, a model-independent estimate. We then argue that these two regimes could be the origin of the gap in X-ray luminosity in the hard state, the wiggles, and different slopes seen in the radio–X-ray correlation, and even the existence of outliers.