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تسجيل مستخدم جديدGRB Probes of the Early Universe with EXIST
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Jonathan E. Grindlay
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With the Swift detection of GRB090423 at z = 8.2, it was confirmed that GRBs are now detectable at (significantly) larger redshifts than AGN, and so can indeed be used as probes of the Early Universe. The proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission has been designed to detect and promptly measure redshifts and both soft X-ray (0.1 - 10 keV) and simultaneous nUV-nIR (0.3 - 2.3microns) imaging and spectra for GRBs out to redshifts z ~18, which encompasses (or even exceeds) current estimates for Pop III stars that are expected to be massive and possibly GRB sources. Scaling from Swift for the ~10X greater sensitivity of EXIST, more than 100 GRBs at z >=8 may be detected and would provide direct constraints on the formation and evolution of the first stars and galaxies. For GRBs at redshifts z >= 8, with Lyman breaks at greater than 1.12microns, spectra at resolution R = 30 or R = 3000 for afterglows with AB magnitudes brighter than 24 or 20 (respectively) within ~3000sec of trigger will directly probe the Epoch of Reionization, formation of galaxies, and cosmic star formation rate. The proposed EXIST mission can probe these questions, and many others, given its unparalleled combination of sensitivity and spatial-spectral-temporal coverage and resolution. Here we provide an overview of the key science objectives for GRBs as probes of the early Universe and of extreme physics, and the mission plan and technical readiness to bring this to EXIST.
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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.
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