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

We present time-dependent models of the remnant accretion disks created during compact object mergers, focusing on the energy available from accretion at late times and the composition of the disk and its outflows. We calculate the dynamics near the outer edge of the disk, which contains the majority of the disk’s mass and determines the accretion rate onto the central black hole. This treatment allows us to follow the evolution over much longer timescales (100 s or longer) than current hydrodynamic simulations. At late times the disk becomes advective and its properties asymptote to self-similar solutions with an accretion rate Ṁd ∝ t −4/3 (neglecting outflows). This late-time accretion can in principle provide sufficient energy to power the latetime 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 disk alone to produce significant accretion power well beyond the onset of the advective phase. Unless the viscosity is quite low (α ∼ < 10), this occurs before the start of observed flaring at ∼ 30 s; continued mass inflow at late times thus appears required to explain the late-time activity from shortduration gamma-ray bursts. We show that the composition of the disk freezes-out when the disk is relatively neutron rich (electron fraction Ye ≃ 0.3). Roughly 10 M⊙ of this neutron-rich material is ejected by winds at late times. During earlier, neutrinocooled phases of accretion, neutrino irradiation of the disk produces a wind with Ye ≃ 0.5, which synthesizes at most ∼ 10 M⊙ of Ni. We highlight what conditions are favorable for Ni production and predict, in the best cases, optical and infrared transients peaking ∼ 0.5 − 2 days after the burst, with fluxes a factor of ∼ 10 below the current observational limits.