UBVRI photometry of SN 1993J in M81: The first 120 days

We present photometry of SN 1993J in M81 (NGC 3031) in the Johnson- Cousins UBVRI system, starting within three days of the probable time of explosion and ending 120 days later. The reddening along the line of sight to this supernova is uncertain-there is evidence for small (E(B- V) ~ 0.08 mag] and moderate [E(B- V) ~ 0.32 mag] values. For each value, we correct the observed fluxes for extinction and calculate the evolution of the sum of all optical flux, which contains roughly half of the supernova's total flux during the period of observation. We also fit the colors to a Planck function, and determine the temperature and total luminosity of the supernova for each value of reddening. A peculiar Type II supernova, SN 1993J gradually evolved spectroscopically to resemble a Type Ib in some respects. We compare its bolometric evolution with that of the prototypical Type Ib SN 1983N, and find the two to be similar.