Light-curve calculations of supernovae from fallback gamma-ray bursts

The currently favored model for long-duration gamma-ray bursts (GRBs) invokes explosions from the collapse of a massive star down to a black hole, either directly or through fallback. Those GRBs forming via fallback will produce much less radioactive nickel, and hence it has been argued (without any real calculation) that these systems produce dim supernovae. These fallback black hole GRBs have recently been argued as possible progenitors of a newly discovered set of GRBs lacking any associated supernovae. Here we present the first ever radiation-hydrodynamics calculations of the light curves produced in the hypernova explosion by a delayed-fallback gamma-ray burst. We find that the bolometric light curve is dominated by shock-deposited energy, not the decay of radioactive elements. As such, observations of such bursts actually probe the density in the progenitor wind more than it does the production of radioactive nickel.