The effect of deep hydrogen burning in the accreted envelope of a neutron star on the properties of X-ray bursts

The thermal and compositional evolution of a neutron star has been numerically followed to determine the long-term properties of X-ray bursts produced by thermonuclear shell flashes in its accreted hydrogen-rich envelope. Uniform burning over the entire neutron star surface is assumed and mass accretion rates greater than ∼0.1Ṁ_Edd (where Ṁ_Edd is the critical mass accretion rate for which the accretion luminosity is equal to the Eddington luminosity) are considered. Specific attention is focused on the consequences of electron capture initiated burning of hydrogen at high densities (≳10⁷ g cm^-3). The degree of heating associated with the burning of the residual hydrogen (i.e., the matter which is not completely processed in the outburst) is a function of the mass accretion rate and the composition of the accreted matter. Heating of the neutron star envelope is found to be more important for greater mass accretion rates and for greater residual hydrogen abundances. Because of the higher envelope temperatures, the resulting bursts are weaker and recur more frequently, for a given mass accretion rate, than in situations where the deep hydrogen burning does not occur. The mass accretion rate, which delineates strong X-ray bursts (where the ratio of the peak burst luminosity to the quiescent level of emission is greater than ∼3) from weak X-ray bursts, lies in the range of 0.1-0.2 times the Eddington value. Weak burst activity is found for accretion rates extending to about the Eddington limit provided that the helium content of the accreted matter is greater than ∼0.23. The implications of our results with regard to the absence of regular, periodic X-ray bursting activity in the bright low-mass X-ray binary sources are briefly discussed.