Hyperaccretion Disks around Neutron Stars

TL;DR

This paper analyzes the structure and cooling efficiency of hyperaccretion disks around neutron stars, comparing simple and elaborate models to understand their physical properties and neutrino luminosity.

Contribution

It introduces a detailed analysis of neutron star hyperaccretion disks, including a comparison between simplified and complex models to assess their consistency and physical insights.

Findings

Neutron star disks cool more efficiently than black hole disks.

Neutron star disks produce higher neutrino luminosity.

Simple and elaborate models yield consistent results.

Abstract

(Abridged) We here study the structure of a hyperaccretion disk around a neutron star. We consider a steady-state hyperaccretion disk around a neutron star, and as a reasonable approximation, divide the disk into two regions, which are called 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 physical properties of the entire disk clearly, we first adopt a simple model, in which some microphysical processes in the disk are simplified, following Popham et al. and Narayan et al. Based on these simplifications, we 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, compared with the black-hole disk, the neutron star disk can cool more efficiently and produce a much higher neutrino luminosity. Finally, we consider an elaborate model with more physical considerations about the thermodynamics and microphysics in the neutron star disk (as recently developed in studying the neutrino-cooled disk of a black hole), and compare this elaborate model with our simple model. We find that most of the results from these two models are basically consistent with each other.