Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection, light curves, and broad-band spectral evolution of an intermediate time flux excess (a “bump ” which is redder in color relative to typical afterglow emission) in the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red bump is remarkably well described by a redshifted Type Ic supernova, with a luminosity about half that of SN 1998bw, which occurred contemporaneously with the gamma-ray burst event. Given the simplicity of the model, we argue against alternative models for intermediate-time bumps (such as dust echoes) which require fine tuning to reproduce the observations. Instead, these results serve as compelling evidence for a massive star origin of long duration gamma-ray bursts. We further exclude the (related) supranova model and discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts. Subject headings: supernovae: general miscellaneous — gamma rays: bursts — supernovae: individua
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