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

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.

ApJL, in press

References in corpus (13)

• Modelling the behaviour of accretion flows in X-ray binaries
• Spectral and temporal properties of Compton scattering by mildly relativistic thermal electrons
• A radio parallax to the black hole X-ray binary MAXI J1820+070
• Properties of AGN coronae in the NuSTAR era II: hybrid plasma
• Hot accretion flow in black hole binaries: a link connecting X-rays to the infrared
• The binary mass ratio in the black hole transient MAXI J1820+070
• The synchrotron boiler and the spectral states of black hole binaries
• On the origin of spectral states in accreting black holes
• Accretion Geometry in the Hard State of the Black-Hole X-Ray Binary MAXI J1820+070
• X-ray dips and a complex UV/X-ray cross-correlation function in the black hole candidate MAXI J1820+070
• The evolution of the broadband temporal features observed in the black-hole transient MAXI J1820+070 with Insight-HXMT
• Analysis of extremely low signal-to-noise ratio data from INTEGRAL/PICsIT
• Observations of Cygnus X-1 in the MeV band by the INTEGRAL imager

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