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. Krühler, D. Malesani, K. Misra, A. Nicuesa Guelbenzu, G. Pugliese, R. Sánchez-Ramírez, S. Schulze, E. R. Stanway, A. de Ugarte Postigo, D. Watson, R. A. M. J. Wijers, D. Xu

*Corresponding author for this work

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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 Ζ=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 × 1052 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 ≈ 5º. The beaming-corrected γ-ray energy is then Eγ ≈ 2 x 1050 erg, while the beaming-corrected kinetic energy is lower, 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
JournalAstrophysical Journal
Issue number2
Number of pages16
Publication statusPublished - 2018


  • Dark ages
  • Reionization
  • First stars
  • Galaxies: high-redshift
  • Gamma-ray burst: general
  • Gamma-ray burst: individual (GRB 120923A)

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