Supernovae, Jets, and Collapsars

We consider the explosion of supernovae and the possible production of avariety of high energy transients by delayed black hole formation in massivestars endowed with rotation. Following the launch of a ``successful'' shock bythe usual neutrino powered mechanism, the inner layers of the star moveoutwards, but lack adequate energy to eject all the matter exterior to theneutron 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, oftenturning it into a black hole and establishing an accretion disk. The accretionrate, about 0.001 to 0.01 solar masses per second, is inadequate to produce ajet mediated by neutrino annihilation, but similar to that invoked inmagnetohydrodynamic (MHD) models for gamma-ray bursts (GRBs). We thus considerthe effect of jets formed by ``fallback'' in stars that are already in theprocess of exploding. We justify a parameterization of the jet power as aconstant times the mass accretion rate, $\epsilon \dot {\rm M} c^2$, andexplore the consequences of $\epsilon$ = 0.001 and 0.01. In supergiants, shockbreakout produces bright x-ray transients that might be a diagnostic of themodel. Jets produced by fallback should be more frequent than those made by theprompt formation of a black hole and may power the most common form ofgamma-ray transient in the universe, although not the most common form seen sofar by BATSE. Those are still attributed to prompt black hole formation, but itmay be that the diverse energies observed for GRBs so far reflect chiefly thevariable collimation of the jet inside the star and a consequently highlyvariable fraction of relativistic ejecta. Indeed, these events may all have acommon total energy near 10$^{52}$ erg.