High Energy X-Ray Imaging Spectrometer - Hexis

HEXIS is a MIDEX-class mission concept for X-ray astronomy. Its objectives are to improve our knowledge of the high energy X-ray sky by increasing the number of sources above 20 keV to >2,000, discovering transient sources such as X-ray novae and gamma-ray bursts, and making spectral and temporal studies of the sources. With mission life >3 years, a I-year all-sky survey sensitivity of similar to 0.3 mCrab (>20 keV), and continuous monitoring of the entire visible sky? HEXIS will provide unprecedented capabilities. Source positions will be determined to accuracies of a few arcmin or better. Spectra will be determined with an energy resolution of a few keV and source variability will be studied on time scales from <1 sec to years. In addition, 10 times more sensitive studies of limited fields will be performed at the same time. Gamma-ray bursts will be detected about 4 times/week at about the same sensitivity as BATSE and the sensitivity to nova-like X-ray transients will be similar to 6 mCrab in one day. HEXIS contains a set of coded mask imagers that use position-sensitive CZT detectors operating from similar to 5 keV to 200 keV. Detector planes are built with 41 cm(2) CZT detector modules which employ crossed-strip readout to obtain a pixel size of 0.5 mm. Nine modules are grouped in a 369 cm(2) array for each imager. In the past 2 years significant progress has been made on techniques required for HEXIS: position-sensitive CZT detectors and ASIC readout, coded mask imaging, and background properties at balloon altitudes. Scientific and technical details of HEXIS are presented together with results from tests of detectors and a coded mask imager.