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Observations show that at least some gamma-ray bursts (GRBs) happen simultaneously with core-collapse supernovae (SNe), thus linking by a common thread nature's two grandest explosions. We review here the growing evidence for and theoretical implications of this association, and conclude that most long-duration soft-spectrum GRBs are accompanied by massive stellar explosions (GRB-SNe). The kinetic energy and luminosity of well-studied GRB-SNe appear to be greater than those of ordinary SNe, but evidence exists, even in a limited sample, for considerable diversity. The existing sample also suggests that most of the energy in the explosion is contained in nonrelativistic ejecta (producing the supernova) rather than in the relativistic jets responsible for making the burst and its afterglow.

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The evolution of rotating stars with zero-age main sequence (ZAMS) masses in the range 8 M ⊙ to 25 M ⊙ is followed through all stages of stable evolution. The initial angular momentum is chosen such that the star's equatorial rotational velocity on the ZAMS ranges from zero to ∼ 70 % of break-up. The stars rotate rigidly on the ZAMS as a consequence of angular momentum redistribution during the pre-main sequence evolution. Redistribution of angular momentum and chemical species are then followed as a consequence of Eddington-Sweet circulation, the Solberg-Høiland instability, the Goldreich-Schubert-Fricke instability, and secular and dynamic shear instability. The effects of the centrifugal force on the stellar structure are included. Convectively unstable zones are assumed to tend towards rigid rotation and uncertain mixing efficiencies are gauged by observations. We find, as noted in previous work, that rotation increases the helium core masses and enriches the stellar envelopes with products of hydrogen burning. We determine, for the first time, the angular momentum distribution in typical presupernova stars along with their detailed chemical structure. Angular momentum loss due to (non-magnetic) stellar winds and the redistribution of angular momentum during core hydrogen burning are of crucial importance for the specific angular momentum of the core. Neglecting magnetic fields, we find angular momentum transport from the core to the envelope to be unimportant after core helium burning. We obtain specific angular momenta for the iron core and overlaying material of 10 16 . . . 10 17 erg s. These values are insensitive to the initial angular momentum and to uncertainties in the efficiencies of rotational mixing. They are small enough to avoid triaxial deformations of the iron core before it collapses, but could lead to neutron stars which rotate close to break-up. They are also in the range required for the collapsar model of gamma-ray bursts. The apparent discrepancy with the measured rotation rates of young pulsars is discussed.

A variety of current models of gamma-ray bursts (GRBs) suggest a common engine : a black hole of several solar masses accreting matter from a disk at a rate of 0.01 to 10 s~1. Using a numerical M _ model for relativistic disk accretion, we have studied steady state accretion at these high rates. Outside about 108 cm, the disk is advection dominated ; energy released by dissipation is carried in by the optically thick gas, and the disk does not cool. Inside this radius, for accretion rates greater than about 0.01 s~1 a global state of balanced power comes to exist between neutrino losses, chieÑy pair capture on M _ nucleons, and dissipation. As a result of these losses, the temperature is reduced, the density is raised, and the disk scale height is reduced compared to the advective solution. The sudden onset of neutrino losses (due to the high temperature dependence) and photodisintegration leads to an abrupt thinning of the disk that may provide a favorable geometry for jet production. The inner disk remains optically thin to neutrinos for accretion rates of up to about 1 s~1. The energy emitted in neutrinos is less, and in M _ the case of low accretion rates, very much less, than the maximum efficiency factor for black hole accretion (0.057 for no rotation ; 0.42 for extreme Kerr rotation) times the accretion rate, Neutrino tem-M 0 c2. peratures at the last stable orbit range from 2 MeV (no rotation, slow accretion) to 13 MeV (Kerr geometry, rapid accretion), and the density ranges from 109 to 1012 g cm~3. The efficiency for producing a pair Ðreball along the rotational axis by neutrino annihilation is calculated and found to be highly variable and very sensitive to the accretion rate. For some of the higher accretion rates studied, it can be several percent or more ; for accretion rates less than 0.05 s~1, it is essentially zero. The efficiency of M _ the Blandford-Znajek mechanism in extracting rotational energy from the black hole is also estimated. In light of these results, the viability of various gamma-ray burst models is discussed, and the sensitivity of the results to disk viscosity, black hole rotation rate, and black hole mass is explored. A diverse range of GRB energies seems unavoidable, and neutrino annihilation in hyperaccreting black hole systems can explain bursts of up to 1052 ergs. Larger energies can be inferred for beaming systems.

As a massive star evolves through multiple stages of nuclear burning on its way to becoming a supernova, a complex, differentially rotating structure is set up. Angular momentum is transported by a variety of classic instabilities, and also by magnetic torques from fields generated by the differential rotation. We present the first stellar evolution calculations to follow the evolution of rotating massive stars including, at least approximately, all these effects, magnetic and non-magnetic, from the zero-age main sequence until the onset of iron-core collapse. The evolution and action of the magnetic fields is as described by Spruit (2002) and a range of uncertain parameters is explored. In general, we find that magnetic torques decrease the final rotation rate of the collapsing iron core by However, with the exception of Spruit & Phinney (1998) and Maeder & Meynet (2004), all studies of massive stellar evolution to date have ignored what is probably a major effect,

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