Optical Evidence for Circumstellar Interaction Around SN 1993J

We study the circumstellar interaction around SN 1993J by its intermediate-band light curves obtained by the 60/90 cm Schmidt telescope at Xinglong station. The optical emission showed a slow decay of 0.05±0.02 mag/100 d in the period from 1995 to 2003, invoking a main energy contribution from SN-circumstellar interaction at late times. The relatively flat power law SN density model fits better with the observations. In particular, the line ratio of O IIIλλ4959, 5007 and Na I D relative to H_α are well reproduced by the model. Moreover, the H_α light curve displayed obvious bump structures at some epochs, which is probably attributed to the density fluctuations in the ambient material that surrounds the reverse shockwave.