The recent hard X-ray surveys performed by INTEGRAL and Swift have started to reveal the demographics of compact sources including Super-Massive Black Holes hosted in AGNs and have proven invaluable in tracking explosive events as the death of massiv
e stars revealed by Gamma-Ray Bursts up to cosmological distances. Whereas the observations have contributed significantly to our understanding of the sources populations in the Local Universe, it has also become evident that revealing the processes that drive the birth and evolution of the first massive stars and galaxies would have required a further big step in both sensitivity and capability to study transient phenomena since their very beginning and covering different wavebands simultaneously. Therefore, after its decennial history as a proposed hard X-ray survey mission, EXIST has now turned into a new, more advanced concept with three instruments on board covering the IR/optical and X-ray/soft gamma-ray bands. The EXIST new design (Grindlay 2009a) is therefore much improved in its capability for prompt study of GRBs (with autonomous determination of the redshift for many of them) and broadband spectral studies of SMBHs and transients in the high energy band from 0.1 to several hundred keV, with sensitive optical/NIR and soft X-ray identifications and followup studies.
The Energetic X-ray Imaging Survey Telescope (EXIST) mission concept is optimized for study of high-z GRBs as probes of the early Universe. With a High Energy Telescope (HET) incorporating a 4.5m^2 5-600keV (CZT; 0.6mm pixels) detector plane for code
d aperture imaging a 90deg x 70deg (>10% coding fraction) field of view with 2 resolution and <20 (90% conf.) positions for >5 sigma sources, EXIST will perform rapid (<200sec) slews onto GRBs. Prompt images and spectra are obtained with a co-aligned soft X-ray telescope (SXI; 0.1 - 10keV) and with a 1.1m optical-IR telescope (IRT) simultaneously in 4 bands (0.3 - 0.52micron, 0.52 - 0.9micron, 0.9 - 1.38micron, and 1.38 - 2.3micron). An initial image (100s) will yield prompt identification within the HET error circle from a <2 prompt SXI position; or from VIS vs. IR dropouts or variability. An autonomous spacecraft re-point (<30) will then put the GRB on a 0.3 x 4 slit for either R = 3000 (for AB <21) or R =30 (for AB ~21-25) prompt spectra over the 0.3 - 0.9 micron and 0.9 - 2.3 micron bands. This will provide onboard redshifts within ~500-2000sec for most GRBs, reaching z ~20 (for Lyman-alpha breaks) if such GRBs exist, and spectra for studies of the host galaxy and local re-ionization patchiness as well as intervening cosmic structure. With ~600 GRBs/yr expected, including ~7-10% expected at z >7, EXIST will open a new era in studies of the early Universe as well as carry out a rich program of AGN and transient-source science. An overview of the GRB science objectives and a brief discussion of the overall mission design and operations is given, and example high-z GRB IRT spectra are shown. EXIST is being proposed to the Astro2010 Decadal Survey as a 5 year Medium Class mission that could be launched as early as 2017.
The synergy of GLAST and the proposed EXIST mission as the Black Hole Finder Probe in the Beyond Einstein Program is remarkable. With its full-sky per orbit hard X-ray imaging (3-600 keV) and nuFnu sensitivity comparable to GLAST, EXIST could measure
variability and spectra of Blazars in the hard X-ray synchrotron component simultaneous with GLAST (~10-100GeV) measures of the inverse Compton component, thereby uniquely constraining intrinsic source spectra and allowing measured high energy spectral breaks to measure the cosmic diffuse extra-galactic background light (EBL) by determining the intervening diffuse IR photon field required to yield the observed break from photon-photon absorption. Such studies also constrain the physics of jets (and parameters and indeed the validity of SSC models) and the origin of the >100 MeV gamma-ray diffuse background likely arising from Blazars and jet-dominated sources. An overview of the EXIST mission, which could fly in the GLAST era, is given together with a synopsis of other key synergies of GLAST-EXIST science.
