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Astronomy & Astrophysics


EDP Sciences


Broad-band optical observations of the extraordinarily bright optical afterglow of the intense gamma-ray burst GRB 991208 started ∼2.1 days after the event and continued until 4 Apr. 2000. The flux decay constant of the optical afterglow in the R-band is −2.30 ±0.07 up to ∼5 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 event, that otherwise would have reached naked eye brightness, implies either a previous additional break prior to ∼2 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 H0 =60 km s−1 Mpc−1, Ω0 =1 and Λ0 = 0), implying an isotropic energy release of 1.15 1053 erg which may be relaxed by beaming by a factor >102. 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 MB = −18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of (11.5 ±7.1) M☉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 ∼3.5 day after the burst (Dec. 12.0) implying a cooling frequency νc below the optical band, i.e. supporting a jet model with p= −2.30 as the index of the power-law electron distribution


Additional authors: H. Pedersen, E. Palazzi, E. Pian, N. Masetti, R. Sagar, V. Mohan, A. K. Pandey, S. B. Pandey, S. N. Dodonov, T. A. Fatkhullin, V. L. Afanasiev, V. N. Komarova, A. V. Moiseev, R. Hudec, V. Simon, T. A. Fatkhullin, V. L. Afanasiev, V. 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. Kouvelioutou, S. Guziy, A. Shlyapnikov, U. Hopp, G. Feulner, S. Dreizler, H. Boehnhardt, J. M. Paredes, J. Marti, E. Xanthopoulos, H. E. Kristen, J. Smoker, and K. Hurley