GRB 011121: A Collimated Outflow into Wind-blown Surroundings
Mitrofanov, I.; Pian, E.; Palazzi, E.; Greiner, J.; Salvato, M.; Kouveliotou, C.; Kaper, L.; Sterken, C.; Hartmann, D. H.; Vreeswijk, P. M.; Wijers, R. A. M. J.; Rol, E.; Masetti, N.; Klose, S.; Stecklum, B.; Gorosabel, J.; Castro-Tirado, A. J.; Tanvir, N. R.; Hjorth, J.; Burud, I.; Castro Cerón, J. M.; Lodieu, N.; Fynbo, J. U.; Litvak, M.; Sanin, A.; Lamer, G.; Schwarz, R.; Fruchter, A.; Andersen, M. I.; Rhoads, J.; Zeh, A.; Scholz, R. D.; Grinkov, V.; van den Heuvel, E.
Germany, United States, Italy, Belgium, Spain, Russia, Denmark, Netherlands, United Kingdom, 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.
Based on observations collected at the European Southern Observatory, La Silla and Paranal, Chile (ESO Programme 165.H-0464).