Time-dependent models of accretion discs formed from compact object mergers

We present time-dependent models of the remnant accretion discs created during compact object mergers, focusing on the energy available from accretion at late times and the composition of the disc and its outflows. We calculate the dynamics near the outer edge of the disc, which contains the majority of the disc's mass and determines the accretion rate on to the central black hole. This treatment allows us to follow the evolution over much longer time-scales (100 s or longer) than current hydrodynamic simulations. At late times the disc becomes advective and its properties asymptote to self-similar solutions with an accretion rate (M) over dot(d) proportional to t(-4/3) (neglecting outflows). This late-time accretion can in principle provide sufficient energy to power the late-time activity observed by Swift from some short-duration gamma-ray bursts. However, because outflows during the advective phase unbind the majority of the remaining mass, it is difficult for the remnant disc alone to produce significant accretion power well beyond the onset of the advective phase. Unless the viscosity is quite low (alpha less than or similar to 10(-3)), this occurs before the start of observed flaring at similar to 30 s; continued mass inflow at late times thus appears required to explain the late-time activity from short-duration gamma-ray bursts. We show that the composition of the disc freezes-out when the disc is relatively neutron rich (electron fraction Y-e similar or equal to 0.3). Roughly 10(-2) M-circle dot of this neutron-rich material is ejected by winds at late times. During earlier, neutrino-cooled phases of accretion, neutrino irradiation of the disc produces a wind with Y-e similar or equal to 0.5, which synthesizes at most similar to 10(-3) M-circle dot of Ni-56. We highlight what conditions are favorable for Ni-56 production and predict, in the best cases, optical and infrared transients peaking similar to 0.5-2 d after the burst, with fluxes a factor of similar to 10 below the current observational limits.