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Issue A&A
Volume 370, Number 2, May I 2001
Page(s) 398 - 406
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010247



A&A 370, 398-406 (2001)
DOI: 10.1051/0004-6361:20010247

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

A. J. Castro-Tirado1, 2, V. V. Sokolov3, 4, J. Gorosabel5, J. M. Castro Cerón6, J. Greiner7, R. A. M. J. Wijers8, B. L. Jensen9, J. Hjorth9, S. Toft9, H. Pedersen9, E. Palazzi10, E. Pian10, N. Masetti10, R. Sagar11, V. Mohan11, A. K. Pandey11, S. B. Pandey11, S. N. Dodonov3, T. A. Fatkhullin3, V. L. Afanasiev3, V. N. Komarova3, 4, A. V. Moiseev3, R. Hudec12, V. Simon12, P. Vreeswijk13, E. Rol13, S. Klose14, B. Stecklum14, M. R. Zapatero-Osorio15, N. Caon15, C. Blake16, J. Wall16, D. Heinlein17, A. Henden18, 19, S. Benetti20, A. Magazzù20, F. Ghinassi20, L. Tommasi21, M. Bremer22, C. Kouveliotou23, S. Guziy24, A. Shlyapnikov24, U. Hopp25, G. Feulner25, S. Dreizler26, D. Hartmann27, H. Boehnhardt28, J. M. Paredes29, J. Martí30, E. Xanthopoulos31, H. E. Kristen32, J. Smoker33 and K. Hurley34

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

(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 $\pm$ 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 $\pm$ 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 H0= 60 km s-1 Mpc-1, $\Omega_{0}= 1$ and $\Lambda_{0}= 0$), implying an isotropic energy release of 1.151053 erg which may be relaxed by beaming by a factor >102. Precise astrometry indicates that the GRB coincides within 0.2 $^{\prime\prime}$ 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.


Key words: gamma rays: bursts -- galaxies: general -- cosmo logy: observations

Offprint request: A. J. Castro-Tirado, ajct@laeff.esa.es

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© ESO 2001


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