Andromeda: A mission to determine the gamma-ray burst distance scale

Andromeda is a wideeld, imaging, hard X-ray/soft gamma-ray instrument capable of detecting gamma-ray bursts (GRBs) a factor 20 times fainter than GRO/BATSE. During 1-year of a two-year mission, it could determine whether GRBs are Galactic or cosmological in origin by searching for an excess of bursts towards the nearby Andromeda galaxy (M31). As a pointed, imaging instrument with sensitivity in the 10|200 keV band signi cantly better than previous coded-aperture instruments, Andromeda is capable of carrying out important secondary science objectives: for example, studying the soft-gamma-repeater and X-ray transient populations of M31 and the Galactic bulge. Andromeda is a coded aperture gamma-ray telescope consisting of a hexagonal coded mask coupled to an alkalihalide imaging scintillation detector, a ight-proven technology adapted from the balloon-borne Caltech Gamma-Ray Imaging Payload (GRIP). The new instrument is optimized for the 10{200 keV band and has 1:5 angular resolution over a 17 FWHM eld of view. Andromeda is designed to be a small, lowcost mission, and draws its design largely from existing instrumentation. Andromeda was submitted to the STEDI program, and will also be proposed as a NASA Small Explorer.