On the specta of X-ray bursters: Expansion and contraction stages

Details On the specta of X-ray bursters: Expansion and contraction stages On the specta of X-ray bursters: Expansion and contraction stages

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...429..340t&link_type=abstract

The Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 429, no. 1, p. 340-355

Astronomy and Astrophysics

Astrophysics

42

Bursts, Contraction, Expansion, Neutron Stars, X Ray Spectra, X Ray Stars, Helium Plasma, Radiative Transfer, Spectral Energy Distribution, Stellar Models

Scientific paper

The theory of spectral formation during the explosion and contraction stages of X-ray bursters, which include the effects of Computonization and free-free absorption and emission, is described. Analytical expressions are provided for color ratios, and the spectral shape is given as a function of input parameters, elemental abundance, neutron star mass 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 analytical expression is derived from the sonic point position and the value of the sonic velcoity. Using this value as a boundary condition at the sonic point, the velocity, density, and temperature profile are calculated throughout the whole photosphere. It is shown that the atmosphere radiates spectra having a low-energy power-law shape and blackbody-like hard tail. In the expansion stage the spectra depend strongly on the temperature of the helium-burning zone at the neutron star surface. The X-ray photosheric radius increases to approximately 100 km or more, depending on the condition of the nuclear burning on the surface of the neutron star in the course of the expansion.

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