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The optical emission of supernovae normally fades beyond detection a few months after explosion. A small number of these events, however, are visible for a few years. While the decline in some cases continues at a slow rate (e.g. SN~1986J and SN~1988Z), there are a few examples of SNe which remain at a constant luminosity for years. The power source of the latter is supposedly the conversion of shock energy into heat and ionization in a dense circumstellar medium left over from an earlier mass loss episode of the progenitor star.
We have monitored SN 1980K in NGC 6946 for the past few years and present broad-band filter light curves for this object over the age range from 10 to 12 years. The brightness of SN~1980K remained constant over this period at R=$21.9\pm0.1$, V=$22.8\pm0.2$, and I=$22.2\pm0.3$. These magnitudes allow to correct the fluxes of observed spectra and improve the analysis of the energy balance emitted by the different lines. The constant luminosity implies a rather steady power source which is almost certainly provided by the shock heating of the circumstellar gas as long as there is not a rapid drop in its density. The total flux at optical wavelengths (B through I) amounts to $1.3\times10^{-14}$ erg s$^{-1}$ cm$^{-2}$. At the distance of NGC 6946 this corresponds to $\sim 9\cdot10^{37} \times (\frac{D_{NGC 6946}}{7.5 Mpc})$ erg s$^{-1}$. The flux of SN~1980K appears to have been constant since its redetection (Fesen \& Becker, 1990, ApJ, 351, 437), and the total energy radiated in the last five years is of the order of $10^{46}$ erg. This is only a tiny fraction of the available kinetic energy in the shock.
The sudden fading observed in SN~1957D has been attributed to the shock reaching the edge of the remnant of the progenitor wind (Long et al., ApJ, 395, 632). The duration of the sustained emission and the shock velocity should eventually provide a sensitive measure of the mass loss episode in a giant star. Monitoring of SN~1980K is currently the most promising candidate for such a study.
