Why did Supernova 1054 shine at late times?

The Crab nebula is the remnant of supernova 1054 (SN 1054). The progenitor of this supernova has, based on nucleosynthesis arguments, been modeled as an 8-10 solar mass star. Here we point out that the observations of the late light curve of SN 1054, from the historical records, are not compatible with the standard scenario, in which the late time emission is powered by the radioactive decay of small amounts of Ni-56. Based on model calculations we quantify this discrepancy. The rather large mass of Ni-56 needed to power the late time emission, 0.06[-0.03,+0.02] solar masses, seems inconsistent with abundances in the Crab nebula. The late light curve may well have been powered by the pulsar, which would make SN 1054 unique in this respect. Alternatively, the late light curve could have been powered by circumstellar interaction, in accordance with scenarios in which 8-10 solar mass stars are progenitors to `dense wind' supernovae.