MODEL ATMOSPHERES AND X-RAY-SPECTRA OF BURSTING NEUTRON-STARS

This paper presents derivation of the model atmosphere equations corresponding to plane-parallel nongray atmospheres of very hot neutron stars in hydrostatic and radiative equilibrium. Model equations take into account free-free opacity on hydrogen and helium ions and angle-averaged Compton scattering on electrons with fully relativistic thermal velocities. General-relativistic corrections in the equations correspond to a nonrotating central star. Nonlinear equation of transfer implements an exact photon redistribution function which traces precisely scattering events even with large photon-electron energy exchange. The equation of transfer is solved with the method of variable Eddington factors, and temperature corrections are given by the linearized equation of radiative equilibrium. Numerical results of this paper include tables of surface fluxes for 20 model atmospheres ranging in effective temperature T(e)* from 8 x 10(6) K to 3 x 10(7) K and in surface gravity log g* = 10(15) cm s-2 or less. All the models consist of hydrogen and helium with the solar number abundance N(He)/N(H) = 0.11. Few models very closely approach the Eddington limit. It is found that the spectra of high-gravity models are practically identical with the blackbody spectrum shifted toward higher energies, whereas models approaching the Eddington limit develop a huge low-energy hump. Therefore, the usual approximation of X-ray burst spectra by blackbody curves gets very poor at the Eddington limit. The effects of cooling by nongray free-free absorption and the Compton heating in the uppermost layers produce a characteristic run of temperature with the Rosseland depth, T(tau-R), which can exhibit a deep minimum above the photosphere and huge rise outward. This effect vanishes in models approaching the Eddington limit, and then the atmosphere gets almost isothermal with T(tau-R) far exceeding the effective temperature T(e)*. The paper also discusses the effects of limb darkening and brightening in true intensity distribution of emerging X-rays.