Spectra of the Expansion Stage of X-Ray Bursts

Abstract

We present an analytical theory of thermonuclear X-ray burst atmosphere structure. Newtonian gravity and diffusion approximation are assumed. Hydrodynamic and thermodynamic profiles are obtained as a numerical solution of the Cauchy problem for the first-order ordinary differential equation. We further elaborate a combined approach to the radiative transfer problem which yields the spectrum of the expansion stage of X-ray bursts in analytical form where Comptonization and free-free absorption-emission processes are accounted for and tau ~ r^{-2} opacity dependence is assumed. Relaxation method on an energy opacity grid is used to simulate radiative diffusion process in order to match analytical form of spectrum, which contains free parameter, to energy axis. Numerical and analytical results show high similarity. All spectra consist of a power-law soft component and diluted black-body hard tail. We derive simple approximation formulae usable for mass-radius determination by observational spectra fitting.

31 pages, 4 figures, accepted by the Astrophysical Journal

References in corpus (1)

• A Remarkable Three Hour Thermonuclear Burst From 4U 1820-30

Cited by in corpus (6)

• On the Dynamics of Suddenly Heated Accretion Disks around Neutron Stars
• The Bursting Behavior of 4U 1728-34: Parameters of a Neutron Star and Geometry of a NS-disk system
• On the nature of the flux variability during an expansion stage of a type I X-ray burst: Constraints on Neutron Star Parameters for 4U 1820-30
• Three Type I X-ray bursts from Cyg X-2: Application of Analytical Models for Neutron Star Mass and Radius Determination
• Model Atmospheres for X-ray Bursting Neutron Stars
• Expanded atmospheres and winds in Type I X-ray bursts from accreting neutron stars