Discovery of the Low-Redshift Optical Afterglow of GRB 011121 and Its Progenitor Supernova SN 2001ke

A light echo around SN 1993J was observed 8.2 yr after explosion by a Hubble Space Telescope Wide Field Planetary Camera 2 observation, adding to the small family of supernovae with light echoes. The light echo was formed by supernova light scattered from a dust sheet that lies 220 pc away from the supernova, 50 pc thick along the line of sight, as inferred from the radius and width of the light echo. The dust inferred from the light echo surface brightness is 1000 times denser than the intercloud dust. The graphite-to-silicate fraction cannot be determined by our BVI photometric measurements. However, a pure graphite model can be excluded on the basis of comparison with the data. With future observations, it will be possible to measure the expansion rate of the light echo, from which an independent distance to M81 can be obtained.