Tanvir_2018_ApJ_865_107.pdf (1.93 MB)
The Properties of GRB 120923A at a Spectroscopic Redshift of z approximate to 7.8
journal contributionposted on 2019-09-24, 14:32 authored by NR Tanvir, T Laskar, AJ Levan, DA Perley, J Zabl, JPU Fynbo, J Rhoads, SB Cenko, J Greiner, K Wiersema, J Hjorth, A Cucchiara, E Berger, MN Bremer, Z Cano, BE Cobb, S Covino, V D'Elia, W Fong, AS Fruchter, P Goldoni, F Hammer, KE Heintz, P Jakobsson, DA Kann, L Kaper, S Klose, F Knust, T Kruehler, D Malesani, K Misra, AN Guelbenzu, G Pugliese, R Sanchez-Ramirez, S Schulze, ER Stanway, A de Ugarte Postigo, D Watson, RAMJ Wijers, D Xu
Gamma-ray bursts (GRBs) are powerful probes of early stars and galaxies, during and potentially even before the era of reionization. Although the number of GRBs identified at z ~> 6 remains small, they provide a unique window on typical star-forming galaxies at that time, and thus are complementary to deep field observations. We report the identification of the optical drop-out afterglow of Swift GRB 120923A in near-infrared Gemini-North imaging, and derive a redshift of z = 7.84 +0.06 -0.12 from Very Large Telescope/X-shooter spectroscopy. At this redshift the peak 15–150 keV luminosity of the burst was 3.2 × 10^52 erg s^−1 , and in this sense it was a rather typical long-duration GRB in terms of rest frame luminosity. This burst was close to the Swift/Burst Alert Telescope detection threshold, and the X-ray and near-infrared afterglow were also faint. We present ground- and space-based follow-up observations spanning from X-ray to radio, and find that a standard external shock model with a constant-density circumburst environment of density n ≈ 4 × 10^−2 cm^−3 gives a good fit to the data. The near-infrared light curve exhibits a sharp break at t ≈ 3.4 days in the observer frame which, if interpreted as being due to a jet, corresponds to an opening angle of θjet ≈ 5° . The beaming-corrected γ-ray energy is then Eγ ≈ 2 x 10^50 erg, while the beaming-corrected kinetic energy is lower, EK ≈ 10^49 erg, suggesting that GRB 120923A was a comparatively low kinetic energy event. We discuss the implications of this event for our understanding of the high-redshift population of GRBs and their identification.
This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with program GO12558. Support for Program number GO12558 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. This work is based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere (ESO), Chile under programme 089.A-0067, and on observations obtained at the Gemini Observatory (acquired through the Gemini Science Archive and processed using the Gemini IRAF package), which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil) and Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. When the data reported here were acquired, UKIRT was operated by the Joint Astronomy Centre on behalf of the Science and Technology Facilities Council of the U.K. We thank Tim Carroll for his assistance in making these observations. Based on data obtained with the VLA under program 12A-394. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Dark Cosmology Centre was funded by the DNRF. The research lea
CitationAstrophysical Journal, 2018, 865 (2)
Author affiliation/Organisation/COLLEGE OF SCIENCE AND ENGINEERING/Department of Physics and Astronomy
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