Detection of a Supernova Signature Associated with Grb 011121

Using observations from an extensive monitoring campaign with the Hubble Space Telescope we present the detection of an intermediate time flux excess which is redder in color relative to the afterglow of GRB 011121, currently distinguished as the gamma-ray burst with the lowest known redshift. The red “bump,” which exhibits a spectral roll-over at ∼7200 Å, is well described by a redshifted Type Ic supernova that occurred approximately at the same time as the gamma-ray burst event. The inferred luminosity is about half that of the bright supernova 1998bw. These results serve as compelling evidence for a massive star origin of long-duration gamma-ray bursts. Models that posit a supernova explosion weeks to months preceding the gamma-ray burst event are excluded by these observations. Finally, we discuss the relationship between spherical core-collapse supernovae and gamma-ray bursts. Subject headings: supernovae: general — gamma rays: bursts — supernovae: individual (SN 1998bw)