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

• Published in: Astronomy and astrophysics (Berlin) 2001-05, Vol.370 (2), p.398-406
• Main Authors: Castro-Tirado, A. J., Sokolov, V. V., Gorosabel, J., Castro Cerón, J. M., Greiner, J., Wijers, R. A. M. J., Jensen, B. L., Hjorth, J., Toft, S., Pedersen, H., Palazzi, E., Pian, E., Masetti, N., Sagar, R., Mohan, V., Pandey, A. K., Pandey, S. B., Dodonov, S. N., Fatkhullin, T. A., Afanasiev, V. L., Komarova, V. N., Moiseev, A. V., Hudec, R., Simon, V., Vreeswijk, P., Rol, E., Klose, S., Stecklum, B., Zapatero-Osorio, M. R., Caon, N., Blake, C., Wall, J., Heinlein, D., Henden, A., Benetti, S., Magazzù, A., Ghinassi, F., Tommasi, L., Bremer, M., Kouveliotou, C., Guziy, S., Shlyapnikov, A., Hopp, U., Feulner, G., Dreizler, S., Hartmann, D., Boehnhardt, H., Paredes, J. M., Martí, J., Xanthopoulos, E., Kristen, H. E., Smoker, J., Hurley, K.
• Format: Article
• Language: English
• Subjects: Astronomia de raigs gamma; cosmo logy: observations; Cosmologia; Cosmology; Esclat de raigs gamma; Galaxies; galaxies: general; Galàxies; Gamma ray astronomy; Gamma ray bursts; Gamma rays; gamma rays: bursts; Raigs gamma
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:20010247

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 e ffect, 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 se cond 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 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 ($11.5 \pm 7.1$) $M_{\odot}$ 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 $\nu_{\rm c}$ below the optical band, i.e. supporting a jet model with $p = -$2.30 as the index of the power-law electron distribution.