The Properties of GRB 120923A at a Spectroscopic Redshift of z ≈ 7.8
Cenko, S. B.; Covino, S.; Sánchez-Ramírez, R.; D'Elia, V.; Greiner, J.; Berger, E.; Cucchiara, A.; Goldoni, P.; Kaper, L.; Fruchter, A. S.; Wijers, R. A. M. J.; Klose, S.; Watson, D.; Fynbo, J. P. U.; Krühler, T.; Nicuesa Guelbenzu, A.; Kann, D. A.; Laskar, T.; Bremer, M. N.; Perley, D. A.; Wiersema, K.; Zabl, J.; Levan, A. J.; Fong, W.; Tanvir, N. R.; Malesani, D.; Cano, Z.; Hjorth, J.; Jakobsson, P.; de Ugarte Postigo, A.; Hammer, F.; Heintz, K. E.; Stanway, E. R.; Pugliese, G.; Schulze, S.; Knust, F.; Xu, D.; Rhoads, J.; Misra, K.; Cobb, B. E.
United Kingdom, United States, Denmark, Germany, Spain, Italy, France, Iceland, Netherlands, India, Israel, China
Abstract
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 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 {θ }jet}≈ 5^\circ . The beaming-corrected γ-ray energy is then {E}γ ≈ 2× {10}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.