GRB 011121: A massive star progenitor

Of the cosmological gamma-ray bursts, GRB 011121 has the lowest redshift, z = 0.36. More importantly, the multicolor excess in the afterglow detected in the Hubble Space Telescope (HST) light curves is compelling observational evidence of an underlying supernova. Here we present near-infrared and radio observations of the afterglow, and from our comprehensive afterglow modeling, we find evidence favoring a wind-fed circumburst medium. Lacking X-ray data, we are unable to conclusively measure the mass-loss rate, M,but obtain an estimate, (M) over dot similar to 2 x 10(-7)/nu(w3) M. yr(-1), M, where nu(w3) is the speed of the wind from the progenitor in units of 10(3) km s(-1). This (M) over dot is similar to that inferred for the progenitor of the Type Ibc supernova SN 1998bw that has been associated M with the peculiar burst GRB 980425. Our data, taken in conjunction with the HST results of Bloom et al., provide a consistent picture: the long-duration GRB 011121 had a massive star progenitor that exploded as a supernova at about the same time as the gamma-ray burst event. Finally, we note that the gamma-ray profile of GRB 011121 is similar to that of GRB 980425.