Model atmospheres and X-ray spectra of iron-rich bursting neutron stars. II. Iron rich Comptonized Spectra

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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