Hyperaccretion Disks around Neutron Stars

It is usually proposed that hyperaccretion disks surrounding stellar-mass black holes at an accretion rate of a fraction of one solar mass per second, which are produced during the mergers of double compact stars or the collapses of massive stars, are central engines of gamma-ray bursts (GRBs). In some origin/afterglow models, however, newborn compact objects are invoked to be neutron stars rather than black holes. Thus, hyperaccretion disks around neutron stars seem to exist in some GRBs. Such disks may also occur in type-II supernovae. In this paper we study the structure of a hyperaccretion disk around a neutron star. We consider a steady-state disk model and divide the disk into two regions, called inner and outer disks. The inner disk satisfies an adiabatic self-similar structure and the outer disk is similar to the outer region of a hyperaccretion disk around a black hole. By using analytical and numerical methods, we explore the size of the inner disk, the radial distributions of the density, temperature and pressure of the whole disk, the mechanisms of energy heating and cooling, and the efficiency of neutrino cooling. We find that, compared with a black-hole disk, the hyperaccretion disk around a neutron star can cool more efficiently and produce a much higher neutrino luminosity. Subject headings: accretion: accretion disks — gamma rays: bursts — neutrinos — stars: neutron