Supernovae, Jets, and Collapsars

We consider the explosion of supernovae and the possible production of a variety of high energy transients by delayed black hole formation in massive stars endowed with rotation. Following the launch of a ``successful'' shock by the usual neutrino powered mechanism, the inner layers of the star move outwards, but lack adequate energy to eject all the matter exterior to the neutron star. Over a period of minutes to hours a variable amount of mass, about 0.1 to 5 solar masses falls back into the collapsed remnant, often turning it into a black hole and establishing an accretion disk. The accretion rate, about 0.001 to 0.01 solar masses per second, is inadequate to produce a jet mediated by neutrino annihilation, but similar to that invoked in magnetohydrodynamic (MHD) models for gamma-ray bursts (GRBs). We thus consider the effect of jets formed by ``fallback'' in stars that are already in the process of exploding. We justify a parameterization of the jet power as a constant times the mass accretion rate, $\epsilon \dot {\rm M} c^2$, and explore the consequences of $\epsilon$ = 0.001 and 0.01. In supergiants, shock breakout produces bright x-ray transients that might be a diagnostic of the model. Jets produced by fallback should be more frequent than those made by the prompt formation of a black hole and may power the most common form of gamma-ray transient in the universe, although not the most common form seen so far by BATSE. Those are still attributed to prompt black hole formation, but it may be that the diverse energies observed for GRBs so far reflect chiefly the variable collimation of the jet inside the star and a consequently highly variable fraction of relativistic ejecta. Indeed, these events may all have a common total energy near 10$^{52}$ erg.