Discovery of a Light Echo from Supernova 2003gd

Abstract

Archival HST/ACS data reveal details of a light echo around SN 2003gd in the galaxy M74, only the fifth supernova around which resolved echoes have been reported. An echo is detected 0."3 from the supernova between PA 250--360, with fainter signal present at a few other position angles. This material lies ~180 pc in front of the supernova, with a thickness of 60--120 pc, and may delineate the disk of M74. This structure has a gas density of 1--2 cm^{-3}, typical of the interstellar medium, however, the dust grains are smaller than those found in our Galaxy, with maximum grain sizes around 0.25micron. Since only one epoch of data exists, in two wavebands and with low signal-to-noise, deeper, annual visits should be made with HST and ground-based adaptive optics telescopes.