Ignition and Cooling of X-Ray Bursts

Abstract

This paper presents a discussion of the nuclear physics aspects important for the modeling of stellar nucleosynthesis. Particular emphasis is given to the thermonuclear runaway on the surface of accreting neutron stars. We will mainly focus on the discussion of the reaction sequence which causes the break-up of the hot CNO-cycles and triggers the runaway and the associated rapid energy release. Emphasis is also given to the nuclear structure aspects which cause the runaway to stall which subsequently will initiate the radiative cooling phase of the X-ray burst. Historically, the field of nuclear astrophysics has been concerned with the interpretation of the observed elemental and isotopic abundance distribution 1) and with the formulation and description of the originating nucleosynthesis processes. 2) - 4) Each of these nucleosynthesis processes can be characterized by a specific signature in luminosity and/or in the resulting abundance distribution. Improved observational instrumentation and detection methods allow detailed observation and analysis of the luminosity curve of rapid explosive stellar events. A direct comparison between the predicted and observed energy release and the time scale of the explosive event will yield information about the temperature, density and hydrodynamical conditions in the stellar explosion. A detailed simulation however requires detailed knowledge and understanding of the nuclear processes driving the explosive event. Of particular interest are thermonuclear explosions initiated by accretion processes in close binary systems. The basic concept of the explosion mechanism seems reasonably well understood but there are still considerable discrepancies between the predicted observables and the actual observations. The proposed mechanism involves binary systems with one (or two) degenerate objects, like white dwarfs or neutron stars and is characterized by the revival of the dormant objects via mass overflow and accretion from the binary companion. This leads to explosive events like novae, type Ia supernovae, X-ray bursts, and X-ray pulsars. The characteristic differences in the luminosity, time scale, and periodicity depend on the accretion rate and on the nature of the accreting object. Low accretion rates lead to a pile-up of unburned hydrogen, causing the ignition of hydrogen burning via pp-chains and CNO-cycles with pycnonuclear enhancements of the reactions after a critical mass layer is attained. On white dwarfs this triggers nova events, and on neutron stars it results in