GRBs and the 511 keV emission of the Galactic bulge

¹ Institut de Physique Nucléaire d'Orsay, IN2P3-CNRS/Université Paris-Sud, 91406 Orsay Cedex, France e-mail: parizot@ipno.in2p3.fr
² DAPNIA/Service d'astrophysique, CEA-Saclay, 91191 Gif-sur-Yvette, France
³ Institut d'Astrophysique de Paris, 98bis Bd Arago, 75014 Paris, France
⁴ Fédération de Recherche, Astroparticule et Cosmologie, Univ. Paris 7, Collège de France, 75231 Paris Cedex 05, France

Received: 20 October 2004
Accepted: 23 November 2004

Abstract

We consider the phenomenology of the 511 keV emission in the Galactic bulge, as recently observed by INTEGRAL, and propose a model in which the positrons are produced by gamma-ray bursts (GRB) associated with mini-starbursts in the central molecular zone (CMZ). We show that the positrons can easily diffuse across the bulge on timescales of ∼10⁷ yr, and that their injection rate by GRBs is compatible with the observed fluxes if the mean time between two GRBs in the bulge is ∼ yr . We also explain the low disk-to-bulge emission ratio by noting that positrons from GRBs in the Galactic disk should be annihilated on timescales of 10⁴ yr in the dense shell of the underlying supernova remnant, after the radiative transition, while the remnants of GRBs occurring in the hot, low-density medium produced by recurrent starbursts in the CMZ become subsonic before they can form a radiative shell, allowing the positrons to escape and fill the whole Galactic bulge. If the mean time between GRBs is smaller than ~ yr, INTEGRAL should be able to detect the (localized) 511 keV emission associated with one or a few GRB explosions in the disk.