Covariance spectrum of MAXI J1820+070: On the nature of the Comptonizing flow

TL;DR

MAXI J1820+070's hard-state X-ray variability is analyzed using energy-dependent covariance and coherence spectra extended to $E$ up to $150$ keV, leveraging Insight-HXMT data. The authors detect a coherence drop above about $30$ keV and, through joint fits of short- and long-timescale covariance with a two-component Comptonization model, reveal distinct electron temperatures: $kT_e$ is higher for the long-timescale component than for the short-timescale one. They interpret this as evidence for two Comptonization regions—a vertically extended inner flow and a larger-radius outer region—illuminated by different seed-photon fields (disk blackbody and synchrotron) and connected via propagating fluctuations. The study shows that the hard X-ray variability is governed by a combination of seed-photon complexity and geometry-driven Te evolution, with the height of the inner region modulating the illumination and producing the observed Te evolution across the hard state. These results place meaningful constraints on the accretion-flow geometry in BHXRBs and demonstrate the value of high-energy timing diagnostics for probing coronae and hot flows.

Abstract

We present an analysis of the covariance spectrum of the black hole X-ray binary MAXI J1820+070 during its hard state. For the first time, we extend coherence and covariance studies into the hard X-ray band up to 150 keV. We detect a clear drop in coherence above 30 keV on both short- and long-timescales relative to the 2-10 keV reference band. To investigate the origin of the coherent variability, we simultaneously fit the short- and long-timescale covariances and the time-averaged spectra with a Comptonization model. Surprisingly, the electron temperature associated with long-timescale variability is significantly higher than that on short timescales. Moreover, the temperature on long timescales remains relatively constant throughout the hard state, whereas the short-timescale temperature evolves with X-ray luminosity. We attribute the drop in coherence to multiple sources of seed photons, i.e., the blackbody and synchrotron photons. The independence between these two photon fields leads to the drop in coherence. To explain the lower electron temperature on short timescales, we propose a two-Comptonization framework in which short-timescale variability arises from a vertically extended central region, while long-timescale variability originates at larger radii. The elevated geometry of the inner region leads to illumination primarily by cooler outer-disk photons, yielding a lower electron temperature. In this case, the evolution of the height of the elevated region could explain the evolution of the electron temperature associated with the coherent variability throughout the hard state.

Figures (7)

• Figure 1: Insight-HXMT light curves (in units of counts per second) of MAXI J1820+070 obtained from the HE (squares, 28–150 keV), ME (triangles, 10–30 keV), and LE (dots, 1–10 keV) detectors. Different colored shaded regions mark the five epochs selected for our spectral and timing analyses.
• Figure 2: Power density spectrum (PDS) of each epoch in the hard state. The color scheme of the observations is consistent with that shown in Figure \ref{['lc']}. Successive spectra are vertically offset by 0.03 for clarity. The PDS are derived from the 2–10 keV light curves obtained with the LE detector of Insight-HXMT. The shaded regions denote the frequency ranges corresponding to the long- and short-timescale broadband noise in each epoch.
• Figure 3: Coherence as a function of energy over the long timescale (left panel) and short timescale (right panel) frequency ranges for our five observation epochs. The coherence spectra are derived from the five epochs of Insight-HXMT data, including the rise and decay of the hard state outburst. The reference band is selected as 2–10 keV.
• Figure 4: Fractional covariance spectra. The blue and red points represent the spectrum of long and short-timescale noise, respectively. The reference band is selected as 2–10 keV.
• Figure 5: The covariance ratio of the five representative epochs of the Insight-HXMT observations during the hard state. The reference band is 2–10 keV. To highlight the differences at high energy, the ratios at different times have been rescaled by multiplying factors of 1.20, 0.67, 0.83, 0.89, and 0.94, respectively. The legend on the top left of the panel indicates the observation time of the covariance ratio in MJD.