A critically important region of the astrophysical spectrum is the hard X-ray band (~ 5-600 keV), which connects the predominantly thermal (x-ray) and non-thermal (gamma-ray) universe. The proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission, which was approved for study in the 1994 NASA solicitation for New Mission Concepts, would conduct the first highsensitivity all-sky imaging survey (5-600keV) with a wide-field coded aperture telescope array.

EXIST has been studied for a range of mission implementations, from a free-flyer Explorer class (EXIST-MIDEX) through a balloon-borne ULDB prototype survey (EXIST-LITE). However the increased sensitivity and coverage needed for a deep hard x-ray (HX) survey mission to follow on the possible deep medium energy (2-10 keV) survey from ABRIXAS and the limited HX survey from Swift demand a very large area, wide-field survey mission concept. This could best be implemented as a Black Hole Finder Probe under NASA's Beyond-Einstein program.

Context for development of EXIST-- Need for Deep Hard X-ray Survey:

• HX band (~10-600 keV) is last band without imaging all sky survey
• Only HX survey from HEAO-A4 (1979) with sensitivity ~1/20 flux of brightest source (Crab)
• Soft x-ray (0.2-2keV) ROSAT survey (1990) with sensitivity ~0.0005 Crab flux
• Pointed HX missions (SIGMA/GRANAT; OSSE/CGRO; HEXTE/RXTE; PDS/BeppoSAX) reveal bright sources: AGN, galactic black holes and transients
• All sky HX monitor and crude imaging (BATSE/CGRO) show variable HX sky
• ASCA and BeppoSAX (1999) reveal large obscured AGN population
• INTEGRAL discovers obscured sources in gal. bulge.
• Swift enables discovery that GRBs are at significant redshift but probably with limited statistics and limited survey sensitivity

• HX band (~5-600 keV) to overlap ABRIXAS and extend through 511 keV
• Sensitivity ~0.0005 Crab flux to match and extend ROSAT and ABRIXAS and reach significant sample size (~10,000) of AGN full sky
• All sky imaging each orbit to monitor source variability and provide transient alerts
• Imaging resolution ~5arcmin (source positions <1arcmin) to avoid confusion limits
• Spectral resolution E/dE ~ 50-100 to resolve continuum features and broad lines

EXIST Mission Development

EXIST was recommended as a high priority mission for the coming decade by the NASA Gamma Ray Program Working Group (GRAPWG). The Structure and Evolution of the Universe Science (SEUS) committee has further recommended that EXIST be considered for the next NASA Strategic Plan.

Accordingly, an EXIST Science Working Group (EXSWG) has been formed to further define the science and mission needs for EXIST. The EXSWG is likely to expand beyond the present initial group as the mission is further defined and considered in the planning process.

EXIST would make use of large area arrays of Cd-Zn-Te (CZT) imaging hard x-ray detectors. Technology development for CZT is progressing rapidly, with major interest for medical x-ray imaging. EXIST Technology development will also include both hardware and software for real-time ``back projection" scanning imaging of each photon at the full data rates expected.