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


Tanvir N R, Laskar T, Levan A J, Perley D A, Zabl J, Fynbo J P U, Rhoads J, Cenko S B, Greiner J, Wiersema K, et al. The Properties of GRB 120923A at a Spectroscopic Redshift of z~7.8. The Astrophysical Journal [Internet]. 2018;865 :107.


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.

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Last updated on 03/08/2019