  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 coded 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.
