Supernovae, Jets, and Collapsars

Abstract

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