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

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), PO Box 03004, 18080 Granada, Spain
² Laboratorio de Astrofísica Espacial y Física Fundamental (LAEFF-INTA), PO Box 50727, 28080 Madrid, Spain
³ Special Astrophysical Observatory of the Russian Academy of Sciences, Karanchai-Cherkessia, Nizhnij Arkhyz, 357147, Russia
⁴ Isaac Newton Institute of Chile, SAO Branch
⁵ Danish Space Research Institute, Copenhagen, Denmark
⁶ Real Instituto y Observatorio de la Armada, Sección de Astronomía, 11110 San Fernando-Naval, Cádiz, Spain
⁷ Astrophysikalisches Institut, Potsdam, Germany
⁸ Department of Physics and Astronomy, SUNY Stony Brook, NY, USA
⁹ Astronomical Observatory, University of Copenhagen, Copenhagen, Denmark
¹⁰ Istituto Tecnologie e Studio Radiazioni Extraterrestri, CNR, Bologna, Italy
¹¹ U. P. State Observatory, Manora Peak, Nainital 263 129, India
¹² Astronomical Institute of the Czech Academy of Sciences, 251 65 Ondrejov, Czech Republic
¹³ Anton Pannekoehk Institut, Amsterdam, The Netherlands
¹⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁵ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
¹⁶ Oxford University, AX1 4AU Oxford, UK
¹⁷ Deutsches Zentrum für Luft- und Raumfahrt, Lilienstrasse 3, 86156 Augsburg, Germany
¹⁸ U. S. Naval Observatory, Flagstaff station, AZ, USA
¹⁹ Universities Space Research Association, Flagstaff station, AZ, USA
²⁰ Centro Galileo Galilei, Canary Islands, Spain
²¹ Universitá di Milano, Dipartimento di Fisica, Via Celoria 16, 20133 Milano, Italia
²² Institut de Radio Astronomie Millimetrique, Grenoble, France
²³ Universities Research Association, SD-50, NASA/MSFC, Hunstville, AL 35812, USA
²⁴ Nikolaev University Observatory, Nikolskaya 24, 327030 Nikolaev, Ukraine
²⁵ Universitäts-Sternwarte, München, Germany
²⁶ University of Tübingen, Tübingen, Germany
²⁷ Clemson University, Department of Physics and Astronomy, Clemson, SC 29634, USA
²⁸ European Southern Observatory, Santiago, Chile
²⁹ Departament dÁstronomia i Meteorologia, Universitat de Barcelona, Avda. Diagonal 647, 08028 Barcelona, Spain
³⁰ Departamento de Física, Escuela Politécnica Superior, Universidad de Jaén, Virgen de la Cabeza 2, 23071 Jaén, Spain
³¹ University of Manchester, Jodrell Bank Observatory, Macclesfield, Cheshire SK11 9DL, UK
³² Harvard -Smithsonian Center for Astrophysics, Harvard, USA
³³ Department of Pure and Applied Physics, Queens University Belfast, University Road, Belfast, BT7 1NN, UK
³⁴ Space Science Laboratory, University of California at Berkerley, USA

Corresponding author: A. J. Castro-Tirado, ajct@laeff.esa.es

Received: 20 December 2000
Accepted: 19 February 2001

Abstract

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 indicates that GRB 991208 is at 3.7 Gpc (for km s^-1 Mpc^-1, and ), 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 M_B = -18.2, well below the knee of the galaxy luminosity function and we derive a star-forming rate of () 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 below the optical band, i.e. supporting a jet model with 2.30 as the index of the power-law electron distribution.