The extraordinarily bright optical afterglow of GRB 991208 and its host galaxy

A.J. Castro-Tirado, V.V. Sokolov, J. Gorosabel, J.M.C. Ceron, J. Greiner, R.A.M.J. Wijers, B.L. Jensen, J. Hjorth, Søren Toft, H. Pedersen, E. Palazzi, E. Pian, N. Masetti, R.N. Sagar, V. Mohan, A.K. Pandey, S.B. Pandey, S.N. Dodonov, T.A. Fatkhullin, V.L. AfanasievV.N. Komarova, A.V. Moiseev, R. Hudec, V. Simon, P. Vreeswijk, E. Rol, S. Klose, B. Stecklum, M.R. Zapatero-Osorio, N. Caon, C. Blake, J. Wall, D. Heinlein, A. Henden, S. Benetti, A. Magazzu, F. Ghinassi, L. Tommasi, M. Bremer, C. Kouveliotou, S. Guziy, A. Shlyapnikov, U. Hopp, G. Feulner, S. Dreizler, D. Hartmann, H. Boehnhardt, J.M. Paredes, J. Marti, E. Xanthopoulos, H.E. Kristen, J. Smoker, K. Hurley

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-rag burst GRB 991208 started similar to2.1 days after the event and continued until 4 Apr. 2000. The flux decay constant of the optic al afterglow in the R-band is -2.30 +/- 0.07 up to similar to5 days. which is very likely due to the jet effect. and it is followed by a much steeper decay with constant -3.2 +/- 0.2. the fastest one ever seen in a GRB optical afterglow. A negative detection in several all-sky films taken simultaneously with the er ent. that otherwise would have reached naked eye brightness, implies either a previous additional break prior to similar to2 days after the occurrence of the GRB (as expected from the jet effect) or a maximum, as observed in GRB 970508. The existence of a second break might indicate a steepening in the electron spectrum or the superposition of two events, resembling GRB 000301C. Once the afterglow emission vanished, contribution of a bright underlying supernova was found on the basis of the late-time R-band measurements. but the light curve is not sufficiently well sampled to rule out a dust echo explanation. Our redshift determination of z = 0.706 indicates that GRB 991208 is at 3.7 Gpc (for H-0 = 60 km s(-1) Mpc(-1), Omega (0) = 1 and Lambda (0) = 0), implying an isotropic energy release of 1.15 10(53) erg which may. be relaxed by beaming by a factor >10(2). Precise astrometry indicates that the GRB coincides within 0.2" with the host galaxy, thus supporting a massive star origin. The absolute magnitude of the galaxy 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) M-circle dot yr(-1), which is much larger than the present-day rate in our Galaxy. The quasi simultaneous broad-band photometric spectral energy distribution of the afterglow was determined similar to3.5 day after the burst (Dec. 12.0) implying a cooling frequency ve below the optical band, i.e. supporting a jet model with p = -2.30 as the index of the power-law electron distribution.
Original languageEnglish
JournalAstronomy & Astrophysics
Volume370
Issue number2
Pages (from-to)398-406
ISSN0004-6361
Publication statusPublished - 2001

Keywords

  • gamma rays : bursts
  • cosmology : observations
  • galaxies : general

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