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Hybrid Comptonization and Electron-Positron Pair Production in the Black-hole X-Ray Binary MAXI J1820+070
Polish Acad Sci, Nicolaus Copernicus Astron Ctr, Bartycka 18, PL-00716 Warsaw, Poland..
CNRS, 9 Av Colonel Roche,BP 44346, F-31028 Toulouse 4, France.;IRAP, 9 Av Colonel Roche,BP 44346, F-31028 Toulouse 4, France.;Univ Toulouse, Toulouse, France.;UPS OMP, Toulouse, France..
Univ Zielona Gora, Inst Phys, Licealna 9, PL-65417 Zielona Gora, Poland..
Univ Lodz, Fac Phys & Appl Informat, Pomorska 149-153, PL-90236 Lodz, Poland..
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2021 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 914, no 1, article id L5Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
American Astronomical Society , 2021. Vol. 914, no 1, article id L5
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-298867DOI: 10.3847/2041-8213/ac0147ISI: 000658481500001Scopus ID: 2-s2.0-85107966934OAI: oai:DiVA.org:kth-298867DiVA, id: diva2:1581271
Note

QC 20210720

Available from: 2021-07-20 Created: 2021-07-20 Last updated: 2022-10-24Bibliographically approved

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Veledina, AlexandraPoutanen, Juri

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