2008ApJ...683..329Z

It is usually proposed that hyperaccretion disks surrounding stellar-mass black holes, with an accretion rate of a fraction of 1 M_☉/s, 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 introduced as neutron stars rather than black holes. Thus, hyperaccretion disks around neutron stars may 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 hyperaccretion disk, and as a reasonable approximation, divide it into two regions, the inner and outer disks. The outer disk is similar to that of a black hole, and the inner disk has a self-similar structure. In order to study the physical properties of the entire disk clearly, we first adopt a simple model, in which some microphysical processes in the disk are simplified, and analytically and numerically investigate the size of the inner disk, the efficiency of neutrino cooling, and the radial distributions of the disk density, temperature, and pressure. We see that the neutron star disk can cool more efficiently and produce a much higher neutrino luminosity than a black hole disk. Finally, we consider an elaborate model with more physical considerations of the thermodynamics and microphysics in the neutron star disk, and compare this elaborate model with our simple model. We find that most of the results of these two models are basically consistent with each other.