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

DOI: 
10.3847/1538-4357/aadba9
Publication date: 
01/10/2018
Main author: 
Tanvir, N. R.
IAA authors: 
Cano, Z.;Kann, D. A.;Sánchez-Ramírez, R.;de Ugarte Postigo, A.
Authors: 
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.;Covino, S.;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.;Sánchez-Ramírez, R.;Schulze, S.;Stanway, E. R.;de Ugarte Postigo, A.;Watson, D.;Wijers, R. A. M. J.;Xu, D.
Journal: 
The Astrophysical Journal
Refereed: 
Yes
Publication type: 
Article
Volume: 
865
Pages: 
107
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.
Database: 
ADS
SCOPUS
URL: 
https://ui.adsabs.harvard.edu/#abs/2018ApJ...865..107T/abstract
ADS Bibcode: 
2018ApJ...865..107T
Keywords: 
dark ages;reionization;first stars;galaxies: high-redshift;gamma-ray burst: general;gamma-ray burst: individual: GRB 120923A;Astrophysics - High Energy Astrophysical Phenomena;Astrophysics - Cosmology and Nongalactic Astrophysics