On detection of gamma-ray bursts from Andromeda

Abstract

If gamma-ray bursts originate in a corona of high velocity neutron star around the Milky Way, it should also be possible to detect them from a similar corona around Andromeda. Adopting a simple model of such a corona, we evaluate the ability of instruments on existing and on some proposed missions to detect an excess of bursts toward Andromeda. We investigate the optimal properties of an instrument designed to detect such an excess. We find that if the bursts radiate isotropically, an experiment with a sampling distance d max > 600 kpc is required in order to detect the expected excess of bursts in the direction of Andromeda in ≈ 1 year of observation. If the bursts are beamed forward and backward along the direction of the neutron star's kick velocity, an experiment with d max > 800 kpc is required in order to detect the expected excess in a similar amount of time. Detection of an excess of bursts toward Andromeda would constitute definitive evidence that the bursts are Galactic in origin. Conversely, the observation of no excess toward Andromeda would strongly suggest that the bursts are cosmological in origin, provided that the observation is made by an instrument at least orders of magnitude more sensitive than BATSE, given current constraints on the BATSE sampling distance.