The Properties of GRB 120923A at a Spectroscopic Redshift of z ≈ 7.8

N. R. Tanvir, T. Laskar, A. J. Levan, D. A. Perley, J. Zabl, J. P. U. Fynbo, J. Rhoads, S. B. Cenko, J. Greiner, K. Wiersema, J. Hjorth, A. Cucchiara, E. Berger, M. N. Bremer, Z. Cano, B. E. Cobb, S. Covino, V. D'Elia, W. Fong, A. S. FruchterP. Goldoni, F. Hammer, K. E. Heintz, P. Jakobsson, D. A. Kann, L. Kaper, S. Klose, F. Knust, T. Kruehler, D. Malesani, K. Misra, A. Nicuesa Guelbenzu, G. Pugliese, R. Sanchez-Ramirez, S. Schulze, E. R. Stanway, A. de Ugarte Postigo, D. Watson, R. A. M. J. Wijers, D. Xu

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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_{-0.12}^{+0.06} from VLT/X-shooter spectroscopy. At this redshift the peak 15-150 keV luminosity of the burst was 3.2x10^52 erg/s, and in fact the burst was close to the Swift/BAT detection threshold. The X-ray and near-infrared afterglow were also faint, and in this sense it was a rather typical long-duration GRB in terms of rest-frame luminosity. 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 with density, n~4x10^-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 ~5 degrees. The beaming corrected gamma-ray energy is then E_gamma~2x10^50 erg, while the beaming-corrected kinetic energy is lower, E_K~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.
Original languageEnglish
Article number107
Number of pages16
JournalAstrophysical Journal
Issue number2
Early online date27 Sep 2018
Publication statusPublished - 1 Oct 2018


  • Astrophysics - High Energy Astrophysical Phenomena
  • Astrophysics - Cosmology and Nongalactic Astrophysics


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