Neutrino Cooled disk in GRB central engine

At the extreme densities and temperatures typical of the central engine of GRBs, the accreting torus is cooled mainly by advection and by neutrino emission. The latter process is dominated by electron and positron capture onto nucleons ($\beta$ reactions). We calculate the reaction rates and the nuclear composition of matter, assuming that the torus consists of helium, eletron-positron pairs, free neutrons and protons. After determining the equation of state and solving for the disk structure for a given initial accretion rate, we subsequently follow its time evolution. We find that, for accretion rates of the order of $10 M_{\odot}$/s, likely typical for the early stages of the accretion event, the disk becomes unstable, giving rise to variable energy output. This instability may play an important role for producing internal shocks.