Kavli Affiliate: P. C. Joss

| First 5 Authors: A. Majczyna, J. Madej, P. C. Joss, A. Rozanska,

| Summary:

This paper presents the set of plane-parallel model atmosphere equations for

a very hot neutron star (X-ray burst source). The model equations assume both

hydrostatic and radiative equilibrium, and the equation of state of an ideal

gas in local thermodynamic equilibrium (LTE). The equation of radiative

transfer includes terms describing Compton scattering of photons on free

electrons in fully relativistic thermal motion, for photon energies approaching

m_e *c^2. Model equations take into account many bound-free and free-free

energy-dependent opacities of hydrogen, helium, and the iron ions, and also a

dozen bound-bound opacities for the highest ions of iron. We solve model

equations by partial linearisation and the technique of variable Eddington

factors. Large grid of H-He-Fe model atmospheres of X-ray burst sources has

been computed for 10^7 < T_eff < 3*10^7 K, a wide range of surface gravity, and

various iron abundances. We demonstrate that the spectra of X-ray bursters with

iron present in the accreting matter differ significantly from pure H-He

spectra (published in an earlier paper), and also from blackbody spectra.

Comptonized spectra with significant iron abundance are generally closer to

blackbody spectra than spectra of H-He atmospheres. The ratio of color to

effective temperatures in our grid always remains in the range 1.2 < T_c/T_eff

< 1.85. The present grid of model atmospheres and theoretical X-ray spectra

will be used to determine the effective temperatures, radii and M/R ratios of

bursting neutron stars from observational data.

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