GRB 011121: A Collimated Outflow into Wind-blown Surroundings1

J.Greiner,2,3 S.Klose,4 M.Salvato,2,3 A.Zeh,4 R.Schwarz,2,5 D.H.Hartmann,6 N.Masetti,7 B.Stecklum,4 G.Lamer,2 N.Lodieu,2 R.D.Scholz,2 C.Sterken,8 J.Gorosabel,9,10 I.Burud,10 J.Rhoads,10 I.Mitrofanov,11 M.Litvak,11 A.Sanin,11 V.Grinkov,11 M.I.Andersen,2 J.M.Castro Cero´n,12 A.J.Castro-Tirado,9,13 A.Fruchter,10 J.U.Fynbo,14 J.Hjorth,15 L.Kaper,16 C.Kouveliotou,17 E.Palazzi,7 E.Pian,18 E.Rol,16 N.R.Tanvir,19 P.M.Vreeswijk,20 R.A.M.J.Wijers,16 and E.van den Heuvel16

1 Based on observations collected at the European Southern Observatory, La Silla and Paranal, Chile (ESO Programme 165.H-0464).
2Astrophysikalisches Institut, D-14482 Potsdam, Germany.
3Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse Postfach 1603, D-85741 Garching, Germany.
4Thüringer Landessternwarte Tautenburg, Karl-Schwarzschild-Observatorium, Sternwarte 5, D-07778 Tautenburg, Germany.
5Universitäts-Sternwarte Göttingen, Geismarlandstrasse 11, D-37083 Göttingen, Germany.
6Department of Physics and Astronomy, Clemson University, 118 Kinard Laboratory, Clemson, SC 29634.
7Istituto di Astrofisica Spaziale e Fisica Cosmica, CNR, Sez. di Bologna, Via Gobetti 101, I-40129 Bologna, Italy.
8Vrije Universiteit Brussel, Astrofysisch Institute, Pleinlaan 2, B-1050 Brussels, Belgium.
9 Instituto de Astrofísica de Andalucía (IAA-CSIC), Apartado Correos 3004, E-18080 Granada, Spain.
10Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,MD21218.
11Space Research Institute, Russian Academy of Sciences, Profsoyusnaya 84/32, 117810 Moscow, Russia.
12Real Instituto y Observatorio de la Armada, Sección de Astronomía, 11.110 San Fernando-Naval (Cádiz), Spain.
13Laboratorio de Astrofísica Espacial y Física Fundamental, Madrid, Spain.
14Institute of Physics and Astronomy, University of Aarhus, Ny Munkegade, DK-8000 Aarhus C, Denmark.
15Astronomical Observatory, Copenhagen University, Juliane Maries Vej 30, DK-2100 Copenhagen, Denmark.
16Universiteit van Amsterdam, Kruislaan 403, NL-1098 SJ Amsterdam, Netherlands.
17National Space Science and Technology Center, SD-50, 320 Sparkman Drive, Huntsville, AL 35805.
18Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34131 Trieste, Italy.
19Department of Physical Sciences, University of Hertfordshire, College Lane, Hatfield Herts AL10 9AB, UK.
20European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile.

Abstract - We report optical and near-infrared follow-up observations of GRB 011121 collected predominantly at ESO telescopes in Chile. We discover a break in the afterglow light curve after 1.3 days, which implies an initial jet opening angle of about 9°. The jet origin of this break is supported by the fact that the spectral energy distribution is achromatic during the first 4 days. During later phases, GRB 011121 shows significant excess emission above the flux predicted by a power law, which we interpret as additional light from an underlying supernova. In particular, the spectral energy distribution of the optical transient approximately 2 weeks after the burst is clearly not of power-law type but can be presented by a blackbody with a temperature of ~6000 K. The deduced parameters for the decay slope and the spectral index favor a wind scenario, i.e., an outflow into a circumburst environment shaped by the stellar wind of a massive gamma-ray burst (GRB) progenitor. Because of its low redshift of z = 0.36, GRB 011121 has been the best example for the GRB-supernova connection until GRB 030329 and provides compelling evidence for a circumburster wind region expected to exist if the progenitor was a massive star.
DOI: 10.1086/379606 -