The Properties of GRB 120923A at a Spectroscopic Redshift of z ≈ 7.8
Journal
Date Issued
2018
Author(s)
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.
•
Hjorth, J.
•
Cucchiara, A.
•
Berger, E.
•
Bremer, M. N.
•
Cano, Z.
•
Cobb, B. E.
•
•
D'Elia, V.
•
Fong, W.
•
Fruchter, A. S.
•
Goldoni, P.
•
Hammer, F.
•
Heintz, K. E.
•
Jakobsson, P.
•
Kann, D. A.
•
Kaper, L.
•
Klose, S.
•
Knust, F.
•
Krühler, T.
•
Malesani, D.
•
Misra, K.
•
Nicuesa Guelbenzu, A.
•
Pugliese, G.
•
•
Schulze, S.
•
Stanway, E. R.
•
de Ugarte Postigo, A.
•
Watson, D.
•
Wijers, R. A. M. J.
•
Xu, D.
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.
Volume
865
Issue
2
Start page
107
Issn Identifier
0004-637X
Ads BibCode
2018ApJ...865..107T
Rights
open.access
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