Multiple Light Echoes from Supernova 1993J
  Using the technique of PSF-matched image subtraction, we have analyzed archival HST/WFPC2 data to reveal details of at least two light-echo structures, including some unknown before now, around SN 1993J in the galaxy M81. In particular, we see one partial sheet of material 81 pc in front of the SN and tilted ~60 deg relative to the disk plane of M81, and another 220 pc in front of the SN, roughly parallel to the disk. The inferred echoing material is consistent with the HI surface density detected in this region of M81's disk; however, these data imply a fragmented covering factor for the echoing structures. We discuss prospects for future (roughly annual) visits by HST to image these and yet undiscovered echoes in the interstellar and circumstellar environment of SN 1993J. 
Ben E.K. Sugerman and Arlin P.S. Crotts (Columbia University)