GRB Probes of the High-z Universe with EXIST (original) (raw)

The Energetic X-ray Imaging Survey Telescope (EXIST) mission concept was selected for further study under the Astrophysics Strategic Mission Concept Study (ASMCS) program. The mission design 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 90 o x 70 o (>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 -0.52µm, 0.52 -0.9 µm, 0.9 -1.38 µm, and 1.38 -2.3 µm). 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 (at ~300sec after GRB trigger) 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 µm and 0.9 -2.3 µm bands. Ambiguous initial identifications will have objective prism spectra for all objects in a 3.75' x 0.75' portion of the 5' x 5' IRT field. This will provide onboard redshifts within ~500-2000sec for most GRBs, reaching z ~20 (for Lyman-α 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. FIGURE 4. Optical path in IRT instrument, showing dichroics for simultaneous VIS+IR imaging or spectroscopy and servo image compensation (tip/tilt mirror) for fine guiding.