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

TL;DR

This study analyzes the covariance spectrum of MAXI J1820+070 in its hard state, revealing different electron temperatures on short and long timescales and suggesting a two-region Comptonization model to explain the variability.

Contribution

It extends covariance and coherence analysis into the hard X-ray band up to 150 keV and introduces a two-region Comptonization model to explain timescale-dependent spectral properties.

Findings

Coherence drops above 30 keV on short and long timescales.

Long-timescale electron temperature remains high and stable.

Short-timescale electron temperature varies with luminosity.

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.