Urca Nuclide Production in Type-I X-ray Bursts and Implications for Nuclear Physics Studies

TL;DR

This study uses astrophysical modeling to identify when high-mass urca nuclei are produced in Type I X-ray bursts, revealing that their formation occurs at lower temperatures than previously assumed, which impacts nuclear physics research planning.

Contribution

It is the first detailed investigation of urca nuclide production timing and temperature dependence in X-ray bursts using the MESA code, challenging prior assumptions about relevant nuclear reaction energies.

Findings

High-mass urca nuclei are produced during hydrogen-burning freeze-out at 0.4-0.6 GK.

Production occurs at lower temperatures than those affecting burst light curves.

This influences the planning of nuclear physics experiments related to urca nuclides.

Abstract

The thermal structure of accreting neutron stars is affected by the presence of urca nuclei in the neutron star crust. Nuclear isobars harboring urca nuclides can be produced in the ashes of Type I X-ray bursts, but the details of their production have not yet been explored. Using the code {\tt MESA}, we investigate urca nuclide production in a one-dimensional model of Type I X-ray bursts using astrophysical conditions thought to resemble the source GS 1826-24. We find that high-mass ($A\geq55$) urca nuclei are primarily produced late in the X-ray burst, during hydrogen-burning freeze-out that corresponds to the tail of the burst light curve. The $\sim0.4$--0.6~GK temperature relevant for nucleosynthesis of these urca nuclides is much lower than the $\sim1$~GK temperature most relevant for X-ray burst light curve impacts by nuclear reaction rates involving high-mass nuclides. The latter temperature is often assumed for nuclear physics studies. Therefore, our findings alter the excitation energy range of interest in compound nuclei for nuclear physics studies of urca nuclide production. We demonstrate that for some cases this will need to be considered in planning for nuclear physics experiments. Additionally, we show that the lower temperature range for urca nuclide production explains why variations of some nuclear reaction rates in model calculations impacts the burst light curve but not local features of the burst ashes.