  Rapid, hyper-Eddington accretion is likely to power the central engines of
gamma-ray bursts (GRBs). In the extreme conditions of densities and
temperatures the accreting torus is cooled by neutrino emission rather than by
radiation. Another important cooling mechanism is the advection of energy into
the central black hole. We compute the time evolution of a neutrino-dominated
disc that proceeds during the burst and investigate the changes in its density
and temperature. The discrimination between short and long bursts is made on
the basis of the different rates of material inflow to the outer parts of the
disc, thus favoring the binary merger scenario for the short GRBs and the
collapsar scenario for the long ones. Within the context of the collapsar
model, we also study the evolution of the photon luminosity of the remnant disc
up to times of the order of 1 day, and discuss its implications for the
production of emission lines in GRB spectra.
