  Both types of long and short gamma ray bursts involve a stage of a
hyper-Eddington accretion of hot and dense plasma torus onto a newly born black
hole. The prompt gamma ray emission originates in jets at some distance from
this 'central engine' and in most events is rapidly variable, having a form of
spikes and subpulses. This indicates at the variable nature of the engine
itself, for which a plausible mechanism is an internal instability in the
accreting flow. We solve numerically the structure and evolution of the
neutrino-cooled torus. We take into account the detailed treatment of the
microphysics in the nuclear equation of state that includes the neutrino
trapping effect. The models are calculated for both Schwarzschild and Kerr
black holes. We find that for sufficiently large accretion rates (> 10 Msun/s
for non-rotating black hole, and >1 Msun/s for rotating black hole, depending
on its spin), the inner regions of the disk become opaque, while the helium
nuclei are being photodissociated. The sudden change of pressure in this region
leads to the development of a viscous and thermal instability, and the neutrino
pressure acts similarly to the radiation pressure in sub-Eddington disks. In
the case of rapidly rotating black holes, the instability is enhanced and
appears for much lower accretion rates. We also find the important and possibly
further destabilizing role of the energy transfer from the rotating black hole
to the torus via the magnetic coupling.
