\def\ul#1{$\underline{\smash{\vphantom{y} \hbox{#1}}}$} \def\etal {{\sl et~al.}}
\bf On the Spectra of X-ray Bursters

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Session 55 -- SN 1993J; High Energy Astrophysics
Oral presentation, Wednesday, 10:30-12:00, Dwinelle 145 Room

[55.04] \bf On the Spectra of X-ray Bursters

L. Titarchuk$^{\dagger}$ (NASA/GSFC)

We have made an attempt to develop a self-consistent model for the emission of X-ray Bursters. The extraordinary changes of color temperature during the burst strongly suggest that the X-ray emission originates at first from the helium flash and subsequently from matter which is pushed by the radiative pressure when the luminosity reaches the Eddington limit. We propose that the expanding plasma required has its origins near the neutron star. Because Compton scattering opacity increases with radius, radiation from deep layers exerts expansion forces on the outer layers. At higher temperatures lower Compton scattering opacities correspond to higher local Eddington luminosities. We construct models of such expanding plasmas by solving simultaneously the Bernoulli equation, the radiative transfer equation and the equation of state which are appropriate for envelope structure with steady-state mass outflow. We obtain analytical solutions for the whole atmospheric structure as a function of the mass and radius of the neutron star and the helium burning temperature. In particular, there are simple formulas describing the position of the sonic point, the sonic velocity and the mass outflow. The present model, when coupled with observations of both continuum and spectral lines, can give information on the mass-radius relationship of neutron stars and the helium burning temperature. \vskip 0.2truein

\noindent $^{\dagger}$ NAS/NRC Senior Research Associate

Wednesday program listing