# The Properties of GRB 120923A at a Spectroscopic Redshift of z approximate to 7.8

Tanvir, NR, Laskar, T, Levan, AJ, Perley, DA, Zabl, J, Fynbo, JPU, Rhoads, J, Cenko, SB, Greiner, J, Wiersema, K, Hjorth, J, Cucchiara, A, Berger, E, Bremer, MN, Cano, Z, Cobb, BE, Covino, S, D'Elia, V, Fong, W, Fruchter, AS , Goldoni, P, Hammer, F, Heintz, KE, Jakobsson, P, Kann, DA, Kaper, L, Klose, S, Knust, F, Kruehler, T, Malesani, D, Misra, K, Guelbenzu, AN, Pugliese, G, Sanchez-Ramirez, R, Schulze, S, Stanway, ER, de Ugarte Postigo, A, Watson, D, Wijers, RAMJ and Xu, D (2018) The Properties of GRB 120923A at a Spectroscopic Redshift of z approximate to 7.8. Astrophysical Journal, 865 (2). ISSN 0004-637X

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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 gsim 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 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 ${\theta }_{\mathrm{jet}}\approx 5^\circ$. The beaming-corrected γ-ray energy is then ${E}_{\gamma }\approx 2\times {10}^{50}$ erg, while the beaming-corrected kinetic energy is lower, ${E}_{{\rm{K}}}\approx {10}^{49}$ 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.