Program: GS-2005B-Q-5

Title:GRB South
PI:GRB Team
Co-I(s):

Abstract

\\ Securing redshifts for a complete sample of Swift GRBs // The new Gamma-Ray Burst (GRB) satellite Swift is now approaching full operations. The nominal 2-year lifetime of Swift offers a unique chance to build an objectively selected and complete sample of GRBs with measured redshifts unlike the current very inhomogeneous set. This sample will give an unbiased view of GRBs (energetics, luminosity function) and their environments (host galaxies, link to star formation). Many redshifts for Swift bursts will be measured with smaller telescopes, although a small but crucial fraction will require an 8-m class telescope. We propose a programme to measure redshifts from absorption lines in these afterglow spectra. GMOS will allow redshift measurements for all GRBs to z~5.5. Very high-z GRBs and brighter afterglows are the targets of other dedicated proposals. This is a rapid response trigger proposal. We will trigger only on objects which do not have redshifts at the time of our trigger and will make all data public within 48 hours of being taken. \\ Gamma-Ray Bursts as Probes // Swift was successfully launched on November 20, 2004 and is now well into commissioning. We propose to uniquely combine four resources -- Gemini South and SOAR on Cerro Pachon, UNC's Panchromatic Robotic Optical Monitoring and Polarimetry Telescopes (PROMPT) on Cerro Tololo, and UNC's Cox Remote Observing Center (CROC) in North Carolina -- to obtain near-simultaneous 360 nm -- 2.4 micron spectra of sufficiently bright GRB afterglows and deep imaging for fainter afterglows. PROMPT, with u'g'r'R$_{\rm c}$i'z'YJH (six simultaneously) spectral coverage beginning only seconds after satellite notification and automated data reduction within minutes, is more than sufficient to ``promptly'' determine if Gemini South and SOAR should be interrupted and if so in what specific way. UNC already has a 3-year commitment from the SOAR Board to interrupt on this rapid timescale. To achieve as rapid of a response as possible with Gemini South, we propose to use whichever of GNIRS or GMOS-S is on at the moment, and simultaneously perform complementary observations with OSIRIS and UNC-built Goodman on SOAR. Finally, the key to pulling all of this off on the rapid timescale will be the CROC, which will be staffed by a trained GRB observer every night once Swift begins regular operations. \\ Gamma-Ray Bursts: A Unique Probe of Reionization and the Highest Redshift Galaxies // While there is now strong evidence that the universe was re-ionized between z~6.5 to 15, the details and astrophysical sources responsible for this process are not understood. Further progress using quasars is hampered by (i) the proximity effect, which prevents an accurate measure of the Ly-alpha optical depth and detection of high-redshift metal absorbers, (ii) the technical difficulty of blind field quasar searches in the near-infrared (NIR), and (iii) the possibility that black hole growth limits quasars to z<10. Against this backdrop, significant progress may come from observing GRBs. In particular, GRBs do not suffer from a proximity effect on galaxtic scales, so they allow us to measure not only the Ly-alpha optical depth but also trace the metallicity of the host galaxy. In addition, GRBs may arise from the earliest generation of stars, and they are detectable (in the gamma-ray and NIR) beyond z~10. Finally, the brightness and simple power-law shape of the afterglow allows us to easily distinguish high-redshift GRBs from local dusty GRBs, and to obtain high S/N spectra. Here we outline an efficient method, using our active TOO programs on a wide range of 2m to 10m telescopes (including Keck/Magellan), to find bursts at z>7, and we request TOO spectroscopy with GNIRS to begin to trace in detail the process of re-ionization and the properties of the highest-redshift galaxies. \\ Probing the high redshift universe with GRBs // Long-duration gamma-rays bursts are enormously luminous and hence potentially extremely important probes of the early universe. They act as beacons for absorption studies of the IGM and their own host galaxies during the era of reionization, and, being associated with the deaths of massive stars, trace global star formation at this early time. The launch of the Swift satellite, combined with a new generation of large-aperture ground-based robotic telescopes, provides the opportunity to systematically search for and study GRBs and their afterglows at very high redshifts from z~5 to z~20. In addition, our candidate selection may also reveal a population of highly reddened afterglows, hitherto rare amongst observed GRBs, which are likely to provide important clues to star formation in dense galactic environments. Gemini is crucial to this endeavour, providing the nIR spectroscopy from which we will determine redshifts and study the physical conditions along the line of sight. \\ What are the Short Duration Gamma-Ray Bursts? // All our recent progress in understanding GRBs applies to the ``long-duration'' (t>2s) class, for which long-lived afterglows have been found. The distinct, separate class of short-duration GRBs remains enigmatic because their afterglows, if they exist at all, are clearly much fainter. The recently launched Swift satellite should provide a higher rate of rapid, precise localisations of short bursts than hitherto. We propose to continue our program of Gemini Quick Response observations to conduct deep imaging searches for short-burst afterglows, with followup spectroscopy and photometry of any that are found. Finding even a single event would therefore open up a new field of astrophysics. \\ Probing DLAs and the ISM of High Redshift Galaxies \\ with Gamma-Ray Burst Afterglows // Gamma-ray burst afterglows --- by virtue of their brightnesses, featureless power-law spectra, location inside galaxies, and transient nature --- provide a complementary tool to quasars in the study of the chemical and dust content in high redshift galaxies. We propose a spectroscopic study of nine (total of 24 hours) GRBs through the Target of Opportunity program on both Gemini telescopes shortly after ($<1$ day) a GRB is localized from space. The primary objectives are (1) to determine the metallicity, relative abundances and dust extinction in the inner disks of GRB host galaxies for comparison with quasar-probed extended galaxy halos, and (2) to study the absorption and emission properties of foreground damped Lyman $\alpha$ (DLA) galaxies discovered in Gemini spectroscopy of afterglows. The proposed moderate-resolution GMOS spectroscopic observations of bright afterglows---from the atmospheric cutoff to $\sim8000$ \AA---are designed to identify new DLAs and provide accurate metal-line column density measurements of the kinematically-resolved ISM of the host galaxies. GNIRSs observations will be especially useful to precisely measure the redshift and study the ISM of the GRB host for the highest $z$ and most dust extinguished bursts.

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