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Register a new userGRB Probes of the High-z Universe with EXIST
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Jonathan E. Grindlay
Publication date
2009
fields
Physics
and research's language is
English
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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 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.
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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.
We review recent results on the high-redshift universe and the cosmic evolution obtained using Gamma Ray Bursts (GRBs) as tracers of high-redshift galaxies. Most of the results come from photometric and spectroscopic observations of GRB host galaxies once the afterglow has faded away but also from the analysis of the GRB afterglow line of sight as revealed by absorptions in their optical spectrum.
In the context stellar reionization in the standard cold dark matter model, we analyze observations at z~6 and are able to draw three significant conclusions with respect to star formation and the state of the intergalactic medium (IGM) at z~6. (1) An initial stellar mass function (IMF) more efficient, by a factor of 10-20, in producing ionizing photons than the standard Salpeter IMF is required at z~6. This may be achieved by having either (A) a metal-enriched IMF with and a lower mass cutoff of >= 30Msun or (B) 2-4% of stellar mass being Population III massive metal-free stars at z~6. While there is no compelling physical reason or observational evidence to support (A), (B) could be fulfilled plausibly by continued existence of some pockets of uncontaminated, metal-free gas for star formation. (2) The volume-weighted neutral fraction of the IGM of <f_HI>_V~ 10^-4 at z=5.8 inferred from the SDSS observations of QSO absorption spectra provides enough information to ascertain that reionization is basically complete with at most ~0.1-1% of IGM that is un-ionized at z=5.8. (3) Barring some extreme evolution of the IMF, the neutral fraction of the IGM is expected to rise quickly toward high redshift from the point of HII bubble percolation, with the mean neutral fraction of the IGM expected to reach 6-12% at z=6.5, 13-27% at z=7.7 and 22-38% at z=8.8.
Gamma-ray bursts (GRBSs) are produced by rare types of massive stellar explosions. Their rapidly fading afterglows are often bright enough at optical wavelengths, that they are detectable up to cosmological distances. Hirtheto, the highest known redshift for a GRB was z=6.7, for GRB 080913, and for a galaxy was z=6.96. Here we report observations of GRB 090423 and the near-infrared spectroscopic measurement of its redshift z=8.1^{+0.1}_{-0.3}. This burst happened when the Universe was only ~4% of its current age. Its properties are similar to those of GRBs observed at low/intermediate redshifts, suggesting that the mechanisms and progenitors that gave rise to this burst about 600 million years after the Big Bang are not markedly different from those producing GRBs ~10 billion years later.
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 massive 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.
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