Hybrid Comptonization and Electron-Positron Pair Production in the Black-Hole X-Ray Binary MAXI J1820+070

TL;DR

This study analyzes X-ray and gamma-ray spectra from the black-hole binary MAXI J1820+070, revealing non-thermal electron distributions and low electron-positron pair abundance, with implications for the source size and plasma conditions.

Contribution

It introduces a detailed spectral analysis showing non-thermal electron tails and constrains pair production, providing new insights into the plasma environment of the black-hole binary.

Findings

Electron distributions have high-energy power-law tails.

Photon spectra exceed the pair production threshold at 511 keV.

Low pair abundance confirmed by absence of annihilation features.

Abstract

We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform analysis of two joint spectra from NuSTAR and INTEGRAL, covering the range of 3--650 keV, and of an average joint spectrum over the rise of the hard state, covering the 3--2200 keV range. The spectra are well modelled by Comptonization of soft seed photons. However, the distributions of the scattering electrons are not purely thermal; we find they have substantial high-energy tails, well modelled as power laws. The photon tail in the average spectrum is detected well beyond the threshold for electron-positron pair production, 511 keV. This allows us to calculate the rate of the electron-positron pair production and put a lower limit on the size of the source from pair equilibrium. At the fitted Thomson optical depth of the Comptonizing plasma, the limit is about 4 gravitational radii. If we adopt the sizes estimated by us from the reflection spectroscopy of $>$20 gravitational radii, the fractional pair abundance becomes much less than unity. The low pair abundance is confirmed by the lack of both an annihilation feature and of a pair absorption cutoff above 511 keV in the average spectrum.