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The observed features of the light curve of SN 1993J, which has two maxima, are shown to be well reproduced by the explosion of a red-supergiant if its H/He envelope mass has been decreased below $\sim 0.5 M_\odot$. The first maximum of the light curve is due to shock heating of the thin envelope, while the second maximum is due to the radioactive decay of $^{56}$Co. From the date of the second maximum, the progenitor's main sequence mass is estimated to be $\sim$ 12--15$M_\odot$. The thin envelope is likely to be the result of a close binary evolution. The mass of $^{56}$Ni synthesized in SN 1993J is $\sim 0.09 M_\odot$. The light curve properties, in particular, the date of the minimum and the decline rate of the tail suggest that substantial $^{56}$Ni was mixed into helium layers as has been predicted for Type Ib/Ic supernovae. We also calculate the light curves of line $\gamma$-rays and hard X-rays from the $^{56}$Ni-$^{56}$Co decays and discuss the possibility of observing hard radiation with the Compton Gamma-Ray Observatory. We show that hard X-rays from the pulsar can be observed with ASCA in $\sim 3$ years if the pulsar luminosity is as high as the Crab Nebula.
We also model the X-ray emissions from SN 1993J which have been observed from its early stages. To identify the X-ray emission mechanism and constrain the still unknown model parameters for SN 1993J, we carry out hydrodynamical calculations of the collision between the supernova ejecta and the circumstellar matter for the parametrized ejecta models. We find that the observed features of X-rays can be accounted for with thermal bremsstrahlung emission from the shock-heated ejecta. To reproduce the observed hardness of the X-ray emission, however, expansion velocities of the ejecta must be fairly high and the density gradient of the ejecta must be relatively shallow. This is consistent with a relatively low mass envelope of the progenitor.
