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

VI. Radiative efficiency and radio–X-ray correlation during four outbursts from GX 339-4

¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 OHA, UK
² Villanova University, Department of Physics, Villanova, PA 19085, USA
e-mail: greg.marcel@cam.ac.uk or gregoiremarcel26@gmail.com
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
⁵ Universitá degli Studi di Palermo, Dipartimento di Fisica e Chimica, Via Archirafi 36, 90123 Palermo, Italy
⁶ INAF/IASF Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy
⁷ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁸ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Univ. Orléans, 18330 Nançay, France

Received: 24 May 2021
Accepted: 27 September 2021

Abstract

The spectral evolution of transient X-ray binaries can be reproduced by an interplay between two flows separated at a transition radius R_J: a standard accretion disk (SAD) in the outer parts beyond R_J 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: R_J and the disk accretion rate Ṁ_in. In this paper we compare the temporal evolution of both R_J and Ṁ_in for the four outbursts. We show that despite the undeniable differences between the time evolution of each outburst, a unique pattern in the Ṁ_in−R_J 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.