The thermonuclear model for X-ray transients

Abstract

One-dimensional numerical models for the accretion of nuclear material on the surface of neutron stars and subsequent thermonuclear ignition resulting in detectable hard gamma ray bursts are examined. The models were developed using the KEPLER computer code for advanced stages of stellar evolution. Nuclear energy generation on the surface of the star was taken to comprise a 19-isotope nuclear reaction network with full coupling for nuclei spanning the range H to Ni-56. Models for time-dependent convection and an equation of state incorporating leptonic contributions of relativity and degeneracy were employed for a reference 1.41 solar mass neutron star with a radius of 14.3 km. The star was assumed to accrete from 1/10 to the 10th or 11th solar masses per year, a rate which is shown to possibly stabilize. He ignition can occur once 10 to the 23rd to 25th g accumulates, resulting in super-Eddington luminosities and a radiatively driven wind observed as a gamma-ray burst.