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+0−0..0612 from VLT/X-shooter spectroscopy. At this redshift the peak 15-150 keV luminosity of the burst was 3.2 × 1052 erg s−1, 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 ≈ 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 ≈ 5 degrees. The beaming corrected γ-ray energy is then Eγ ≈ 2 × 1050 erg, while the beaming-corrected kinetic energy is lower, EK ≈ 1049 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 language||English (US)|
|State||Published - Mar 27 2017|
- Dark ages, reionization, first stars
- Galaxies: high-redshift
- Gamma-ray burst: general
- Gamma-ray burst: individual (GRB 120923A)
ASJC Scopus subject areas