Gamma-ray Burst Localization Capabilities of the HEXIS High Energy X-Ray All-Sky Survey/Monitor

HEXIS (High Energy X-Ray Imaging Spectrometer) is a concept for MIDEX-class mission to continuously survey nearly the entire sky in the 10 to 200 keV range and make sensitive 2-10 keV observations of selected regions with focusing X-ray telescopes. It uses arrays of position-sensitive CZT detectors and coded masks to achieve ~ 20 arcmin resolution and a 5 sigma sensitivity (>20 keV) of ~ 4 milliCrab in 1 day and 0.2 milliCrab in 1 year. With these capabilities ~ 5,000 sources (non-GRBs) will be discovered and localized, and have their spectra and variability characterized. The sensitivity to gamma-ray bursts will be similar to BATSE, and hundreds of bursts will be detected each year and localized to <20 arcmin. With rapid on-board image processing, the positions would be immediately relayed to earth for follow-up observations. They would also be used to reorient the HEXIS spacecraft and point its focusing X-ray telescopes at the burst positions to make sensitive measurements of the X-ray afterglow and obtain very accurate positions with few arcsec uncertainties. The refined positions would then be relayed to the earth. The HEXIS concept is described and some expected results are presented.