The High Energy Telescope on EXIST: Hunting High Redshift GRBs and other Exotic Transients

The High Energy Telescope (HET) on EXIST is designed to locate high redshift Gamma-Ray Bursts (GRBs) and other rare transients fast (<10 sec) and accurately (< 20′′) in order to allow rapid (<1–2 min) follow-up observations with onboard X-ray/optical/IR imaging and spectroscopy. The HET employs coded-aperture imaging with a 4.5 m 2 imaging CZT detector array and hybrid tungsten mask. The wide energy band coverage (5–600 keV) is optimal for capturing these transients and highly obscured AGN. The continuous scan with the wide field of view (90o × 70o at 10% coding fraction) increases the chance of capturing rare elusive events such as soft Gamma-ray repeaters and tidal disruption events of stars by dormant supermassive black holes. Sweeping nearly the entire sky every two orbits (3 hour), EXIST will also establish a finely-sampled long-term history of the X-ray variability of many X-ray sources, opening up a new time domain for variability studies. In light of the new EXIST design concept, we review the observing strategy to maximize the science return and report on our recent balloon flight test of a prototype for the CZT detectors needed for HET.