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

Andromeda is a wide-field, imaging, hard x-ray/soft gamma-ray instrument capable of detecting gamma-ray bursts (GRBs) a factor approximately 20 times fainter than GRO/BATSE. During one-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 significantly 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 alkali- halide imaging scintillation detector, a flight-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 degree angular resolution over a 17 degree FWHM field of view. Andromeda is designed to be a small, low-cost 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.