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Title: The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy
Author: Castro-Tirado, A. J.
Sokolov, V. V.
Castro Cerón, J. M.
Zapatero-Osorio, M. R.
Caon, N.
Blake, C.
Wall, J.
Benetti, S.
Magazzù, A.
Ghinassi, F.
Paredes i Poy, Josep Maria
Martí Ribas, Josep
Xanthopoulos, E.
Kristen, H. E.
Smoker, J.
Hurley, K.
Keywords: Astronomia de raigs gamma
Raigs gamma
Esclat de raigs gamma
Gamma ray astronomy
Gamma rays
Gamma ray bursts
Issue Date: 2001
Publisher: Springer Verlag
Abstract: Observations of the extraordinarily bright optical afterglow (OA) of GRB 991208 started 2.1 d after the event. The flux decay constant of the OA in the R-band is -2.30 +/- 0.07 up to 5 d, which is very likely due to the jet effect, and after that it is followed by a much steeper decay with constant -3.2 +/- 0.2, the fastest one ever seen in a GRB OA. A negative detection in several all-sky films taken simultaneously to the event implies either a previous additional break prior to 2 d after the occurrence of the GRB (as expected from the jet effect). The existence of a second break might indicate a steepening in the electron spectrum or the superposition of two events. Once the afterglow emission vanished, contribution of a bright underlying SN is found, but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our determination of z = 0.706 indicates that GRB 991208 is at 3.7 Gpc, implying an isotropic energy release of 1.15 x 10E53 erg which may be relaxed by beaming by a factor > 100. Precise astrometry indicates that the GRB coincides within 0.2' with the host galaxy, thus given support to a massive star origin. The absolute magnitude is M_B = -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of 11.5 +/- 7.1 Mo/yr. The quasi-simultaneous broad-band photometric spectral energy distribution of the afterglow is determined 3.5 day after the burst (Dec 12.0) implying a cooling frequency below the optical band, i.e. supporting a jet model with p = -2.30 as the index of the power-law electron distribution.
Note: Versió postprint del document publicat a:
It is part of: Astronomy and Astrophysics, 2001, vol. 370, p. 398-406
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ISSN: 0004-6361
Appears in Collections:Articles publicats en revistes (Física Quàntica i Astrofísica)

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