# The properties of GRB 120923A at a spectroscopic redshift of z=7.8

## TL;DR

This paper reports the discovery and detailed analysis of a gamma-ray burst at redshift 7.8, providing insights into early universe star formation and the properties of high-redshift GRBs.

## Contribution

First spectroscopic confirmation of a GRB at z=7.8, with comprehensive multi-wavelength follow-up and modeling of its afterglow and jet properties.

## Key findings

- GRB 120923A occurred at z=7.84, one of the highest redshifts for GRBs.
- The burst had a peak luminosity of 3.2x10^52 erg/s, close to detection threshold.
- The jet opening angle was estimated to be about 5 degrees.

## 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 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.