2005A&A...432..889P

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 ∼10⁷yr, and that their injection rate by GRBs is compatible with the observed fluxes if the mean time between two GRBs in the bulge is ∼8x(10⁴yr)x(E_GRB/10⁵¹erg). 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 ≲10⁴yr 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 ∼ 0⁴E₅₁yr, INTEGRAL should be able to detect the (localized) 511keV emission associated with one or a few GRB explosions in the disk.