Urca nuclide production in Type-I X-ray bursts and implications for nuclear physics studies

The thermal structure of accreting neutron stars is affected by the presence of urca nuclei in the neutron star crust. Nuclear isobars harbouring 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 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 >= 55) 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 similar to 0.4-0.6 GK temperature relevant for the nucleosynthesis of these urca nuclides is much lower than the similar to 1 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.

Automated summary

This study investigates the production of urca nuclides in Type I X-ray bursts using the MESA code, revealing that high-mass urca nuclei are mainly produced late in the burst and at lower temperatures relevant for nucleosynthesis. The findings suggest a need to consider the lower temperature range for urca nuclide production in nuclear physics studies and planning for experiments.