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

Autor: 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., Kruehler, T., Malesani, D., Misra, K., Guelbenzu, A. Nicuesa, Pugliese, G., Sanchez-Ramirez, R., Schulze, S., Stanway, E. R., Postigo, A. de Ugarte, Watson, D., Wijers, R. A. M. J., Xu, D.
Rok vydání: 2017
Předmět:
Druh dokumentu: Working Paper
DOI: 10.3847/1538-4357/aadba9
Popis: 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.
Comment: 20 pages
Databáze: arXiv
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