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

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 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 ∼0.4–0.6 GK temperature relevant for nucleosynthesis of these urca nuclides is much lower than the ∼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.

Improved Nuclear Physics Near $A=61$ Refines Urca Neutrino Luminosities in Accreted Neutron Star Crusts

“…The resulting reaction rates (see Table I) are compared to rates commonly adopted in astrophysics model calculations in Figure 2. At the ∼0.5 GK temperatures relevant for urca nuclide production [47], our 60 Cu(p, γ) reaction rate is nearly ten times larger than these other rate estimates. Our 61 Zn(p, γ) rate generally falls between the Hauser-Feshbach rates and the shell-model rate of Ref.…”

“…[21] where 60 Cu(p, γ) was found to have an impact and the singlezone model calculations of Ref. [22] are designed to mimic burning in the hottest regions of a multi-zone X-ray burst (where most of the X-ray flux originates), whereas high-A urca nuclide production happens at shallower depths [47]. Focusing on only the hottest zone effectively results in a different flow of the rp-process and therefore a different reaction rate sensitivity.…”

“…[22], which were calculated for a singleset of astrophysical conditions (similar but not identical to Model A) with the code KEPLER [62,63]. Tentative comparisons between results from X-ray burst calculations performed with MESA and KEPLER show general agreement [47,64], but rigorous benchmarking has not been performed to date. (Ong).…”

Single neutron transfer on 23Ne and its relevance forthepathway ofnucleosynthesis in astrophysical X-ray bursts

“…3. Such temperatures are only briefly reached for standard X-ray burst model calculations [6,56] and, in ruling out the previously possible lowertemperature onset of the (α, p) process [14,20], we may now conclude that the pathway through the 22 Mg(α, p) reaction is not relevant for anything but the most energetic bursters.…”

The Impact of Neutron Transfer Reactions on Heating and Cooling of Accreted Neutron Star Crusts

“…For example the free neutron abundance at the 108 Se branchpoint will depend sensitively on the initial abundance distribution that directly effects the release of free neutrons. There are many nuclear uncertainties in the rp-process (Parikh et al 2008;Cyburt et al 2016), in particular in the production of the odd-A nuclei (Merz & Meisel 2021) that largely determines neutron emission. In addition, the nuclear structure effects that lead to a concentration and a "lock-in" of abundances in a small number of nuclei near the neutron drip line due to the impact of nuclear structure on nuclear masses are not well understood and experimental data are lacking.…”