EXIST: The Ultimate Spatial/Temporal Hard X-ray Survey

The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed mission to conduct an all-sky imaging hard x-ray (HX) survey (-5-600 keV) with ~0.05mCrab sensitivity (5a; 6mo.; ~5-100keV) comparable to the ROSAT soft x-ray survey, and to provide the maximum sensitivity and resolution (spatial and temporal) HX imager as the Next Generation GRB mission. Its primary science goals are to i) identify and measure obscured AGN and constrain the accretion luminosity of the universe as well as the cosmic IR background from Blazar spectra coincident with GeV-TeV observations, ii) measure spectra, variability and locations for the faintest GRBs to study the most energetic events in the universe and the earliest epoch of star formation, and iii) study black holes on all scales, from x-ray transients to luminous AGN. EXIST would incorporate a very large area (~8m) imaging Cd-Zn-Te detector and coded aperture telescope array with nearly half-sky instantaneous view which images the full sky each orbit. With fixed zenith pointing, it could be mounted on the ISS or a free flyer and would complement both GLAST and Constellation-X science if launched before 2010, as recommended by the Astronomy and Astrophysics Decadal Survey. REVEALING THE OBSCURED HARD X-RAY UNIVERSE At energies >5-10 keV, the buried nuclei of active galaxies, which provide most of the cosmic x-ray background, become increasingly clear (if not Compton thick) and reveal the accretion power of the universe [1]. The hard x-ray (HX) band provides the most direct view of the central regions of AGN and near-horizon views of black holes; lower energies are often absorbed and y-ray emission is likely converted to pairs in the most compact regions. The energy band from -5-600 keV, which covers the transition from the thermal x-ray universe of hot gas traced by ubiquitous Fe line emission (6.46.7keV) to the extremes of pair plasmas (SllkeV) and the non-thermal universe, is rich in phenomena and yet remains relatively obscure in realization. The sky has not yet been surveyed in this band with the sensitivity or imaging resolution achieved in 1991 with ROSAT for the soft x-ray band. The last full-sky HX survey, carried out with HEAO-1 in 1979 [2] was sensitive to sources only down to -1/20 of the brightest (Crab, CygX-1) for uncrowded high latititude fields, and had only coarse angular CP587, GAMMA 2001: Gamma-Ray Astrophysics 2001, edited by S. Ritz et al. © 2001 American Institute of Physics 0-7354-0027-X/01/$18.00