We show the hydrodynamical evolution of massive accretion disks around black holes, formed when a neutron star is disrupted by a black hole in a binary system. The time dependence is followed in two dimensions assuming azimuthal symmetry for 0.2 seconds, using an ideal gas equation of state. The disk evolves because of the transport of angular momentum due to viscosity. We estimate the energy released in neutrinos, as well as through magnetic-dominated mechanisms, and find it can be as high as E{sub}v ≈10{sup}52 erg and E{sub}(BZ)≈10{sup}51 erg respectively, in approximately 0.1-0.2 seconds. Of the former, only a small fraction (a few per cent) would potentially be capable of producing a burst through vv annihilation, while in principle the latter could do so almost entirely. Thus these systems could in principle account for the energetics of short gamma ray bursts.
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