The thermonuclear model for X-ray transients

The thermonuclear evolution of a 1.41 M/sub sun/ neutron star accreting both solar and metal-deficient mixtures of hydrogen, helium, and heavy elements at rates ranging from about 10/sup -11/ to 10/sup -10/ M/sub sun/ per year is examined using a one-dimensional numerical model. The metal-deficient compositions may result either from placement of the neutron star in a binary system with a Population II red giant or from gravitational settling of heavy ions in the accreted material. For such accretion rates and metallicities, hydrogen burning, mediated by the ..beta..-limited CNO cycle, is stable and leads to the accumulation of the thick helium layer with mass 10/sup 23/--10/sup 25/ g and temperature 0.7< or =T/sub 8/< or =1.2. Helium ignition occurs under extremely degenerate circumstances and is catastrophically violent. In the lower mass helium shells this runaway is propagated as a convective deflagration; for the thicker layers a detonation front is set up which steepens into a strong relativistic shock wave in the neutron star envelope. In all models, greatly super-Eddington luminosities in the outer layers of the neutron star lead to a sustained epoch of radiatively driven mass loss. Observationally, such models may correspond to rapid X-ray transients. The hopeless prospect for constructing a one-dimensional model for ..gamma..-ray bursts without magnetic field confinement is discussed, and uncertainties are pointed out in strong screening correction for the helium burning reaction.