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Accretion by remnant black holes
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Black hole - Disc accretion
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Short, but very bright, bursts of γ - radiation (GRBs), lasting from fractions of seconds to many minutes, are detected on average once a day and have been shown to originate from cosmological distances. The bimodal duration distribution of GRBs indicates two sub-classes: a short, dim and spectrally hard class with typical durations of 0.1s-0.2s, and a long, bright and spectrally soft variety with time scales of 3s-200s. All detailed observations involve only the long type of GRB; the short ones might harbour further surprises.

The currently most favoured central engine mechanism involves a stellar mass black hole accreting material from a disc of matter at nuclear density. Such a configuration comes about as the result of a merger between two neutron stars (NS-NS) or of a neutron star and a black hole (NS-BH), as well as from supernova explosions of very massive rotating stars (referred to as hypernovae or collapsars). Due to the larger masses involved, more energy is available, a priori, in the hypernova scenario; together with its intrinsic time scales of seconds to minutes this makes them natural candidates for the long class of GRBs. Merging compact objects, on the other hand, have dynamical time scales of milliseconds, and are a more appropriate model fo the short GRBs. Additionally, the expected merger rate of NS-NS and NS-BH binaries match the observed rate of GRBs, contrary to the rate of hypernova explosions which is highly uncertain.

The non-thermal spectra of GRBs leads directly to fireball models having very high Lorentz factors and very low baryonic content (typically less than 10^-4 M_o). Thus any central engine scenario has to include a mechanism to extract the available energy; i.e., kinetic/ thermal/potential energy of disc and rotational energy of the black hole, into baryon-poor regions: current models involve neutrinos and/or magnetic fields for this transfer. The energy extracted from the disc or black hole is stored briefly in the form of neutrinos and/or magnetic fields and then emerges concentrated in a highly relativistic jet which is powered by neutrino-antineutrino annihilation or magnetic Poynting flux.