Systematic Study of the Composition of Type I X-Ray Burst Ashes: Neutron Star Structure versus Reaction Rate Uncertainties

Abstract

In this study, we calculate for the first time the impacts of neutron star (NS) structure on the type I X-ray burst ashes using the MESA code. We find an increased mass fraction of the heavier elements with increasing surface gravity (increased mass or decreased radius), resulting in a higher average mass number ( A _ash ) of burst ashes (except for higher-mass NSs due to the competition between the envelope temperature and the recurrence time). The burst strength ( α ) increases as the surface gravity increases, which indicates the positive correlation between A _ash and α with changes in surface gravity. If the α value is higher, heavier p -nuclei should be produced by the type I X-ray burst nucleosynthesis. Besides, the effects of various burst input parameters, e.g., base heating ( Q _b ), metallicity ( Z ), and some new reaction rates, are calculated for comparison. We find that the heavier nucleus synthesis is inversely correlated with the base heating/metallicity; the smaller the base heating/metallicity, the greater the mass fraction of the heavier elements. The α value decreases as Q _b or Z decreases, which also indicates the positive correlation between A _ash and α with variation in Q _b or Z . The new reaction rates from the ( p , γ ) reactions on ^17 F, ^19 F, ^26 P, ^56 Cu, and ^65 As and the ( α , p ) reaction on ^22 Mg have only minimal effects on burst ashes. In hydrogen-rich X-ray binary systems, nuclei heavier than ^64 Ge are fertile, produced with larger NS mass, smaller NS radius, smaller base heating, and smaller metallicity.

Keywords

• Neutron stars
• Low-mass x-ray binary stars
• X-ray bursts
• Reaction rates