Spectroscopic and photometric observations of SN1987A – VI. Days 617 to 792

We present spectroscopic and UBV(RI)CJHKLM photometric observations of SN1987A in the Large Magellanic Cloud made between days 617 and 792 after the Kamiokande-II neutrino event. During the period from day 520 to day 630 the U to M bolometric flux went through a very steep decline, returning after day 670 to the same decline rate as before day 500. Various explanations for these changes are examined. The addition of the supernova output at high energies (derived from models of X-ray and γ-ray data) to that from the U to M spectral region leaves a deficit from the predicted radioactive decay energy after day 500. This deficit can apparently be accounted for if the far–infrared dust emission is also added to the energy budget. There is no compelling evidence for the presence of any additional energy source, such as a pulsar, during this period. However, the errors on the total luminosity are sufficiently high that the presence of a source radiating at a level less than ${10}^{38}\,\text{erg s}^{- 1}$ cannot be excluded.