Nuclear γ-ray emission from very hot accretion flows

¹ Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany
e-mail: ervin.kafexhiu@mpi-hd.mpg.de
² Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, UK
³ National Research Nuclear University MEPhI, Kashirskoje Shosse, 31, 115409 Moscow, Russia
⁴ Department of Physics and Astronomy, Purdue University, West Lafayette, IN 47907-2036, USA
⁵ Astrophysical Big Bang Laboratory, RIKEN, 351-0198 Saitama, Japan
⁶ Space Research Institute of the Russian Academy of Sciences (IKI), 84/32 Profsoyuznaya Str, Moscow 117997, Russia

Received: 25 July 2018
Accepted: 5 February 2019

Abstract

Optically thin accretion plasmas can reach ion temperatures T_i ≥ 10¹⁰ K and thus trigger nuclear reactions. Using a large nuclear interactions network, we studied the radial evolution of the chemical composition of the accretion flow toward the black hole and computed the emissivity in nuclear γ-ray lines. In the advection dominated accretion flow (ADAF) regime, CNO and heavier nuclei are destroyed before reaching the last stable orbit. The overall luminosity in the de-excitation lines for a solar composition of plasma can be as high as few times 10⁻⁵ the accretion luminosity (Ṁc²) and can be increased for heavier compositions up to 10⁻³. The efficiency of transformation of the kinetic energy of the outflow into high energy (≥100 MeV) γ-rays through the production and decay of π⁰-mesons can be higher, up to 10⁻² of the accretion luminosity. We show that in the ADAF model up to 15% of the mass of accretion matter can “evaporate” in the form of neutrons.