Refereed journal article or data article (A1)
Hybrid Comptonization and Electron-Positron Pair Production in the Black-hole X-Ray Binary MAXI J1820+070
List of Authors: Zdziarski Andrzej A., Jourdain Elisabeth, Lubiński Piotr, Szanecki Michał, Niedźwiecki Andrzej, Veledina Alexandra, Poutanen Juri, Dziełak Marta A., Roques Jean-Pierre
Publisher: IOP PUBLISHING LTD
Publication year: 2021
Journal: Astrophysical Journal Letters
Journal name in source: ASTROPHYSICAL JOURNAL LETTERS
Journal acronym: ASTROPHYS J LETT
Article number: ARTN L5
Volume number: 914
Issue number: 1
Number of pages: 9
ISSN: 2041-8205
DOI: http://dx.doi.org/10.3847/2041-8213/ac0147
Self-archived copy’s web address: https://research.utu.fi/converis/portal/detail/Publication/66420557
We study X-ray and soft gamma-ray spectra from the hard state of the accreting black-hole binary MAXI J1820+070. We perform an 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 modeled 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 modeled 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.
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