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

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}<SUB>-0.12</SUB><SUP>+0.06</SUP> from Very Large Telescope/X-shooter spectroscopy. At this redshift the peak 15-150 keV luminosity of the burst was 3.2 × 10<SUP>52</SUP> erg s<SUP>-1</SUP>, 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<SUP>-2</SUP> cm<SUP>-3</SUP> 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 }<SUB>jet</SUB>}≈ 5^\circ . The beaming-corrected γ-ray energy is then {E}<SUB>γ </SUB>≈ 2× {10}<SUP>50</SUP> erg, while the beaming-corrected kinetic energy is lower, {E}<SUB>{{K</SUB>}}≈ {10}<SUP>49</SUP> 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.