Hyperaccreting Neutron-Star Disks and Neutrino Annihilation

Newborn neutron stars surrounded by hyperaccreting and neutrino-cooled disks may exist in some gamma-ray bursts (GRBs) and/or supernovae (SNe). In this paper we further study the structure of such a neutron-star disk based on the two-region (i.e., inner & outer) disk scenario following our previous work, and calculate the neutrino annihilation luminosity from the disk in various cases. We investigate the effects of the viscosity parameter α, energy parameter ε (measuring the neutrino cooling efficiency of the inner disk) and outflow strength on the structure of the entire disk as well as the effect of emission from the neutron star surface boundary emission on the total neutrino annihilation rate. The inner disk satisfies the entropy-conservation self-similar structure for the energy parameter ε ≃ 1 and the advection-dominated structure for ε < 1. An outflow from the disk decreases the density and pressure but increases the thickness of the disk. Moreover, compared with the black-hole disk, the neutrino annihilation luminosity above the neutron-star disk is higher, and the neutrino emission from the boundary layer could increase the neutrino annihilation luminosity by about one order of magnitude higher than the disk without boundary emission. The neutron-star disk with the advection-dominated inner disk could produce the highest neutrino luminosity while the disk with an outflow has the lowest. Although a heavily mass-loaded outflow from the neutron star surface at early times of neutron star formation prevents the outflow material from being accelerated to a high bulk Lorentz factor, an energetic ultrarelativistic jet via neutrino annihilation can be produced above the stellar polar region at late times if the disk accretion rate and the neutrino emission luminosity from the surface boundary layer are sufficiently high. Subject headings: accretion: accretion disks — black holes — gamma rays: bursts — neutrinos — stars: neutron