The Multi-Messenger Astrophysics of the Galactic Centre Proceedings IAU Symposium No. 322, 2016 R.M. Crocker, S.N. Longmore & G.V. Bicknell, eds. c  International Astronomical Union 2017 doi:10.1017/S1743921316012096 Sgr A* as Source of the Positrons Observed in the Galactic Center Region Pierre Jean 1 , Nidhal Guessoum 2 and Katia Ferri` ere 1 1 Universit´ e de Toulouse; UPS-OMP; IRAP; Toulouse, France & CNRS: IRAP, 9 Av. colonel Roche, BP 44346, F-31028 Toulouse cedex 4, France 2 American University of Sharjah, Physics Department, PO Box 26666, Sharjah, UAE emails: pjean@irap.omp.eu, nguessoum@aus.edu, Katia.Ferriere@irap.omp.eu Abstract. We explore the possibility that a substantial fraction of the positrons observed to annihilate in the central region of our Galaxy come from the supermassive black hole Sgr A* that lies at the center. This idea was proposed by several authors, but the propagation of the emitted positrons into the bulge and beyond remained a serious problem for models of the origin of GC positrons. We assume models of positron production with different energies. The propagation of positrons from their production site is followed in detail with Monte-Carlo simulations, taking into ac- count the physical conditions of the propagation regions as well as various physical interactions. Using the known physics of positron annihilation in astrophysical environments, we calculate the properties of the annihilation emission (time evolution and spatial distribution) for the different models under consideration. We present the results of these simulations and the conclusions/constraints that can be in- ferred from them. Keywords. Positron Annihilation, Galactic Center, Gamma Ray Lines, Sgr A* 1. Introduction Positron annihilation radiation has been observed for almost forty years now (see the full review by Prantzos et al., 2010, and recently Siegert et al. 2016). Its flux and diffuse distribution, with a high concentration in the Galactic bulge, implies a rate of annihilation (and, in steady state, an equal rate of production) of positrons of ∼ 10 43 per second. Determining the source(s) of positrons needed to explain this rate of annihilation as well as the observed spatial distribution has been a challenge, however. Indeed, positrons are known (Guessoum et al. 1991 and others) to live long (∼ 10 5 years in the ISM), and thus to travel far from their birth sites and die in regions where matter is more concentrated. Source distributions hence differ from annihilation maps. The specific hypothesis we wish to explore here is whether positrons from the su- permassive black hole (SMBH) Sgr A* could propagate in and fill the bulge. The idea of positrons being produced (in various ways) by the SMBH was proposed by several authors (Cheng et al., 2006; Titarchuk & Chardonnet 2006; Totani 2006; Cheng et al., 2007). Here we assume a collisional propagation of positrons and simulate their lives and deaths taking into account the physical conditions of the region (density, temperature, ionization, and magnetic field) and the main physical interactions until annihilation. 172 https://www.cambridge.org/core/terms. https://doi.org/10.1017/S1743921316012096 Downloaded from https://www.cambridge.org/core. IP address: 54.88.26.180, on 14 Dec 2021 at 12:19:38, subject to the Cambridge Core terms of use, available at