ON THE SPECTRA OF X-RAY BURSTERS - EXPANSION AND CONTRACTION STAGES

The theory of spectral formation during the explosion and contraction stages of X-ray bursters, which include the effects of Comptonization and free-free absorption and emission, is described. Analytical expres sions are provided for color ratios, and the spectral shape is given a s a function of input parameters, elemental abundance, neutron star ma ss and radius, and Eddington ratio. An Eulerian calculation is used to determine the photospheric evolution accurately during the Eddington luminosity phase. The developed analytical theory for hydrodynamics of the expansion takes into account the dependence of Compton scattering opacity on electron temperature. An analytic expression is derived fr om the sonic point position and the value of the sonic velocity. Using this value as a boundary condition at the sonic point, the velocity, density, and temperature profile are calculated throughout the whole p hotosphere. It is shown that the atmosphere radiates spectra having a low-energy power-law shape and blackbody-like hard tail. In the expans ion stage the spectra depend strongly on the temperature of the helium -burning zone at the neutron star surface. The X-ray photospheric radi us increases to approximately 100 km or more, depending on the conditi on of the nuclear burning on the surface of the neutron star in the co urse of the expansion.