Neutrino Trapping and Accretion Models for Gamma‐Ray Bursts

Many models of Gamma Ray Bursts invoke a central engine consisting of a blackhole of a few solar masses accreting matter from a disk at a rate of a fractionto a few solar masses per second. Popham et al. and Narayan et al. have shownthat, for Mdot >~ 0.1 Msun/s, accretion proceeds via neutrino cooling andneutrinos can carry away a significant amount of energy from the inner regionsof the disks. We improve on these calculations by including a simpleprescription for neutrino transfer and neutrino opacities in such regions. Wefind that the flows become optically thick to neutrinos inside a radiusR~6-40R_s for Mdot in the range of 0.1 -10 Msun/s, where R_s is the black holeSchwarzchild radius. Most of the neutrino emission comes from outside thisregion and, the neutrino luminosity stays roughly constant at a value L_{\nu} ~10^{53} erg/s. We show that, for Mdot > 1 Msun/s, neutrinos are sufficientlytrapped that energy advection becomes the dominant cooling mechanism in theflow. These results imply that neutrino annihilation in hyperaccreting blackholes is an inefficient mechanism for liberating large amounts of energy.Extraction of rotational energy by magnetic processes remains the most viablemechanism.Comment: 23 pages, added reference, corrected typo and bremsstrahlung formul