Tracking the evolution of the accretion flow in MAXI J1820+070 during its hard state with the JED-SAD model

¹ Università degli Studi di Palermo, Dipartimento di Fisica e Chimica, via Archirafi 36, 90123 Palermo, Italy
e-mail: alessio.marino@unipa.it
² INAF/IASF Palermo, via Ugo La Malfa 153, 90146 Palermo, Italy
³ IRAP, Université de Toulouse, CNRS, UPS, CNES, Toulouse, France
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK
⁶ Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
⁷ CNRS, IRAP, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ Université de Toulouse, CNES, UPS-OMP, 31028 Toulouse, France

Received: 20 April 2021
Accepted: 31 August 2021

Abstract

Context. X-ray binaries in outburst typically show two canonical X-ray spectral states (i.e., hard and soft states), as well as different intermediate states, in which the physical properties of the accretion flow are known to change. However, the truncation of the optically thick disk and the geometry of the optically thin accretion flow (corona) in the hard state are still debated. Recently, the JED-SAD paradigm has been proposed for black hole X-ray binaries, aimed at addressing the topic of accretion and ejection and their interplay in these systems. According to this model, the accretion flow is composed of an outer standard Shakura-Sunyaev disk (SAD) and an inner hot jet emitting disk (JED). The JED produces both hard X-ray emission, effectively playing the role of the hot corona, and radio jets. The disruption of the JED at the transition to the soft state coincides with the quenching of the jet.

Aims. In this paper we use the JED-SAD model to describe the evolution of the accretion flow in the black hole transient MAXI J1820+070 during its hard and hard-intermediate states. Unlike the previous applications of this model, the Compton reflection component has been taken into account.

Methods. We use eight broadband X-ray spectra, including NuSTAR, NICER, and the Neil Gehrels Swift Observatory data, providing a total spectral coverage of 0.8–190 keV. The data were directly fitted with the JED-SAD model. We performed the procedure twice, considering two different values for the innermost stable circular orbit (ISCO): 4 R_G (a_* = 0.55) and 2 R_G (a_* = 0.95).

Results. Our results suggest that the optically thick disk (the SAD) does not extend down to the ISCO in any of the considered epochs. In particular, assuming R_ISCO = 4 R_G, as the system evolves toward the transitional hard-intermediate state, we find an inner radius within a range of ∼60 R_G in the first observation down to ∼30 R_G in the last one. The decrease of the inner edge of the SAD is accompanied by an increase in the mass-accretion rate. However, when we assume R_ISCO = 2 we find that the mass accretion rate remains constant and the evolution of the accretion flow is driven by the decrease in the sonic Mach number m_S, which is unexpected. In all hard–intermediate state observations, two reflection components, characterized by different values of ionization, are required to adequately explain the data. These components likely originate from different regions of the SAD.

Conclusions. The analysis performed provides a coherent physical evolution of the accretion flow in the hard and hard-intermediate states and supports a truncated disk scenario. We show that a flared outer disk could, in principle, explain the double reflection component. The odd results obtained for R_ISCO = 2 R_G can also be considered as further evidence that MAXI J1820+070 harbors a moderately spinning black hole, as suggested in other works.