Influence of Nuclear Reaction Rate Uncertainties on Neutron Star Properties Extracted from X-Ray Burst Model–Observation Comparisons

Abstract

Type-I X-ray bursts can be used to determine properties of accreting neutron stars via comparisons between model calculations and astronomical observations, exploiting the sensitivity of models to astrophysical conditions. However, the sensitivity of models to nuclear physics uncertainties calls into question the fidelity of constraints derived in this way. Using X-ray burst model calculations performed with the code MESA, we investigate the impact of uncertainties for nuclear reaction rates previously identified as influential and compare them to the impact of changes in astrophysical conditions, using the conditions that are thought to best reproduce the source GS 1826-24 as a baseline. We find that reaction rate uncertainties are unlikely to significantly change conclusions about the properties of accretion onto the neutron star surface for this source. However, we find that reaction rate uncertainties significantly hinder the possibility of extracting the neutron star mass-radius ratio by matching the modeled and observed light curves due to the influence of reaction rates on the modeled light curve shape. Particularly influential nuclear reaction rates are 15 O(α, γ) and 23 Al(p, γ), though other notable impacts arise from 14 O(α, p), 18 Ne(α, p), 22 Mg(α, p), 24 Mg(α, γ), 59 Cu(p, γ), and 61 Ga(p, γ). Furthermore, we find that varying some nuclear reaction rates within their uncertainties has an impact on the neutron star crust composition and thermal structure that is comparable to relatively significant changes accretion conditions. arXiv:1812.07155v1 [astro-ph.HE]

Spallation-altered Accreted Compositions for X-Ray Bursts: Impact on Ignition Conditions and Burst Ashes

“…These larger CNO abundances, especially at lower accretion rates, alter the amount of hydrogen present at the time of burst ignition.We show that this has a significant effect on X-ray burst model calculations of the composition of the burst ashes. Our results are therefore important to take into account in Xray burst model calculations that are used to predict the thermal and compositional structure of accreted neutron star crusts (Meisel et al 2019;Lau et al 2018).…”

“…Between Z = 10 −5 and 10 −14 , the mass fraction of isotopes summed by mass number (X(A)) can vary by more than a factor of 2 ( Figure 6, lower panel). The lower Z calculations show particularly differences in the oscillation pattern associated with the A = 4n (Meisel et al 2019) since the ignition conditions are even less helium-rich. These changes are significant relative to other changes in astrophysical conditions and nuclear reaction rates (Meisel et al 2019).…”

“…For reduced Z, the abundance distribution is shifted to higher mass fractions. This is expected, as reduced CNO abundances reduce hydrogen burning during accretion, leading to more hydrogen rich conditions at burst ignition and therefore extended hydrogen burning (Heger et al 2007;José et al 2010;Meisel et al 2019). Between Z = 10 −5 and 10 −14 , the mass fraction of isotopes summed by mass number (X(A)) can vary by more than a factor of 2 ( Figure 6, lower panel).…”

Status of the Mg24(α,γ)Si28 reaction rate at stellar temperatures

“…In summary, the available nuclear data on 28 Si relevant to the 24 Mg(α, γ) 28 Si reaction have been reviewed and the reaction rate recalculated on the basis of new level tion rate does not significantly alter the calculated X-ray burst light curve. Therefore, this reaction rate, one of eight identified as important for X-ray burst modelobservation comparisons [3], is adequately constrained for the purposes of determining properties of accreting neutron star systems from such comparisons.…”

“…The factor of ten increase in the reaction rate in Refs. [1,3] was chosen as a plausible uncertainty for a nuclear reaction rate involving a relatively high leveldensity compound nucleus. However, there are existing evaluations of the 24 Mg(α, γ) 28 Si reaction rate from the STARLIB collaboration [7] and from experimental studies [4] which have much smaller uncertainties.…”