The inner flow geometry in MAXI J1820+070 during hard and hard-intermediate states

Abstract

Context. We present a systematic X-ray spectral-timing study of the recently discovered, exceptionally bright black hole X-ray binary system MAXI J1820+070. Our analysis focuses on the first part of the 2018 outburst, covering the rise throughout the hard state, the bright hard and hard-intermediate states, and the transition to the soft-intermediate state. Aims. We address the issue of constraining the geometry of the innermost accretion flow and its evolution throughout an outburst. Methods. We employed two independent X-ray spectral-timing methods applied to archival NICER data of MAXI J1820+070. We first identified and tracked the evolution of a characteristic frequency of soft X-ray thermal reverberation lags (lags of the thermally reprocessed disc emission after the irradiation of variable hard X-ray photons). This frequency is sensitive to intrinsic changes in the relative distance between the X-ray source and the disc. Then, we studied the spectral evolution of the quasi-thermal component responsible for the observed thermal reverberation lags. We did so by analysing high-frequency covariance spectra, which single out spectral components that vary in a linearly correlated way on the shortest sampled timescales and are thus produced in the innermost regions of the accretion flow. Results. The frequency of thermal reverberation lags steadily increases throughout most of the outburst, implying that the relative distance between the X-ray source and the disc decreases as the source softens. However, near transition this evolution breaks, showing a sudden increase (decrease) in lag amplitude (frequency). On the other hand, the temperature of the quasi-thermal component in covariance spectra, due to disc irradiation and responsible for the observed soft reverberation lags, consistently increases throughout all the analysed observations. Conclusions. This study proposes an alternative interpretation to the recently proposed contracting corona scenario. Assuming a constant height for the X-ray source, the steady increase in the reverberation lag frequency and in the irradiated disc temperature in high-frequency covariance spectra can be explained in terms of a decrease in the disc inner radius as the source softens. The behaviour of thermal reverberation lags near transition might be related to the relativistic plasma ejections detected at radio wavelengths, suggesting a causal connection between the two phenomena. Throughout most of the hard and hard-intermediate state, the disc is consistent with being truncated (with an inner radius Rin ≳ 10 Rg), reaching close to the innermost stable circular orbit only near transition.

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

“…This difference with respect to the results of Y21 is due to (1) the inner disk as- sumed at the ISCO in Y21 while estimated from the spectral fits in the present work, and (2) taking into account the spectral complexity of this source in the present work, with the presence of two Comptonization regions, the softer one dominating at softer X-rays, and a harder one at harder X-rays. Such stratification is also indicated by the presence of hard lags in this source, i.e., harder X-rays delayed with respect to softer ones (De Marco et al 2021), and by spectral-timing modelling of Kawamura et al (2021). Spectral complexity of this source was also found by Buisson et al (2019), who fitted the NuSTAR spectra with reflection of two lampposts at different heights.…”

“…It was found that the reflection fraction, defined as the ratio of the coronal flux that illuminates the disk to that emitted outside, showed an overall decrease with the decreasing hardness of the X-ray spectrum. The latter de-crease, in turn, was found (Kara et al 2019;De Marco et al 2021) to be strongly correlated with the decreasing time lag of soft X-rays with respect to hard ones observed by the Neutron star Interior Composition ExploreR (NICER; Gendreau et al 2016). Y21 interpreted this effect as indicating that the corona is relativistically outflowing with the outflow velocity increasing with the decreasing coronal scale height.…”

“…In our view, X-ray spectral models cannot by themselves be taken as fully reflecting the physical reality. They provide only some approximations, which then can be tested by combining with results from timing amd other studies (see, e.g., De Marco et al 2021). We also note that while the Fe abundance has to remain constant, some changes of the fitted inclinations are possible due to precession and warping of various parts of the reflecting disk.…”

The X-ray spectral-timing contribution of the stellar wind in the hard state of Cyg X-1

“…Wilkinson & Uttley 2009). In addition, the disc is observed to lead the variations of the Comptonisation component on the long timescales (> 1 s, Uttley et al 2011), while, for some sources, it is observed to respond to hard X-ray variability on the shortest sampled timescales (< 1 s) (Uttley et al 2011;De Marco et al 2015;Kara et al 2019;De Marco et al 2021). The former behaviour is usually interpreted in terms of inward propagation of mass accretion rate fluctuations in a spectrally inhomogeneous medium (Lyubarskii 1997;Kotov et al 2001;Arévalo & Uttley 2006;Ingram & van der Klis 2013;Mushtukov et al 2018;Bollimpalli et al 2020), while the latter is ascribed to the thermal response of the disc to variable hard X-ray irradiation (thermal reverberation, e.g.…”

“…The former behaviour is usually interpreted in terms of inward propagation of mass accretion rate fluctuations in a spectrally inhomogeneous medium (Lyubarskii 1997;Kotov et al 2001;Arévalo & Uttley 2006;Ingram & van der Klis 2013;Mushtukov et al 2018;Bollimpalli et al 2020), while the latter is ascribed to the thermal response of the disc to variable hard X-ray irradiation (thermal reverberation, e.g. Uttley et al 2011;De Marco et al 2015;De Marco & Ponti 2016;De Marco et al 2017;Kara et al 2019;Wang et al 2020;De Marco et al 2021). In this section, we analyse the X-ray cross-spectral timing properties of Cyg X-1 in order to study the correlations and causal relationship between the disc and the primary Comptonisation components, and estimate the effects of the wind on such properties.…”