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

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 RG (a* = 0.55) and 2 RG (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 RISCO = 4 RG, as the system evolves toward the transitional hard-intermediate state, we find an inner radius within a range of ∼60 RG in the first observation down to ∼30 RG 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 RISCO = 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 mS, 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 RISCO = 2 RG can also be considered as further evidence that MAXI J1820+070 harbors a moderately spinning black hole, as suggested in other works.

Insight-HXMT, NuSTAR and INTEGRAL Data Show Disk Truncation in the Hard State of the Black-Hole X-Ray Binary MAXI J1820+070

“…The presence of a disk extending almost to the ISCO in its hard state was advocated in Kara et al (2019), Buisson et al (2019) and Wang et al (2021). On the other hand, studies by Wang et al (2020), Axelsson & Veledina (2021), Dziełak et al (2021), De , Marino et al (2021), Zdziarski et al (2021b) and Zdziarski et al (2021c), hereafter Z21c, found the disk to be significantly truncated over various stages of the hard state. You et al (2021), hereafter Y21, analyzed broad energy spectra (2-200 keV) from Hard X-ray Modulation Telescope (Insight-HXMT; Zhang et al 2014) for MAXI J1820+070 in the 2018 outburst.…”

The accretion flow geometry of MAXI J1820+070 through broadband noise research with Insight-HXMT

“…The similar results are also obtained from spectral analysis. Meanwhile, there are some other studies that support the truncated accretion disk argument from either spectral analysis or timing analysis (De Marco et al 2021;Marino et al 2021;Zdziarski et al 2021a,b). De found that the frequency of thermal reverberation lags increases steadily, and, on the other hand, the temperature of the quasi-thermal component grows as the source softens, which can be explained in terms of a decrease in the disc inner radius.…”

“…It seems to be contrary to our usual understanding that harder photon comes from inner region. As a result, as Rapisarda et al (2017b); Mushtukov et al (2018) and other paper discuss (Marino et al 2021), we may consider more sophisticated accretion flow geometry (such as bending wave, viscous diffusion, outward fluctuation propagation effect, hot jet-emitting disk etc.) and multi-wavelength observations to explain all the results.…”

Accretion Scenario of MAXI J1820+070 during 2018 Outbursts with Multi-mission Observations

“…Generally, the reflection models assume a primary spectrum, either a broken powerlaw emissivity (in case of extended corona) or an isotropic point-like source at a given height (lamp-post corona) on the rotation axis of the BH. There are studies related to reflection features in the spectra of MAXI J1820+070 on a selected number of observations using various flavors of reflection models (Buisson et al 2019;Chakraborty et al 2020;Buisson et al 2021;Marino et al 2021;You et al 2021;Kawamura et al 2022).…”

“…There are many studies (Shidatsu et al 2018;Buisson et al 2019;Kara et al 2019;Muñoz-Darias et al 2019;Paice et al 2019;Stiele & Kong 2020;Mudambi et al 2020;Wang et al 2020;Tetarenko et al 2021;Bhargava et al 2021;Wang et al 2021;Paice et al 2021;Zdziarski et al 2021a;Bhargava et al 2021;Dziełak et al 2021;Ma et al 2021;Rodi et al 2021;Marino et al 2021;Mastroserio et al 2021;Kawamura et al 2022) that address various spectral and temporal properties of the source. But these works mainly focus on the LHS of the source.…”