Constraints on Bygone Nucleosynthesis of Accreting Neutron Stars

Abstract

Nuclear burning near the surface of an accreting neutron star produces ashes that, when compressed deeper by further accretion, alter the star's thermal and compositional structure. Bygone nucleosynthesis can be constrained by the impact of compressed ashes on the thermal relaxation of quiescent neutron star transients. In particular, Urca cooling nuclei pairs in nuclear burning ashes, which cool the neutron star crust via neutrino emission from e − -capture/β − -decay cycles, provide signatures of prior nuclear burning over the ∼century timescales it takes to accrete to the e − -capture depth of the strongest cooling pairs. Using crust cooling models of the accreting neutron star transient MAXI J0556-332, we show that this source likely lacked Type I X-ray bursts and superbursts 120 years ago. Reduced nuclear physics uncertainties in rp-process reaction rates and e − -capture weak-transition strengths for low-lying transitions will improve nucleosynthesis constraints using this technique.

Neutron transfer reactions in accreting neutron stars

“…For example, if nuclei with A = 28 are present in the mixture with mass fraction ∼ 1% (which is reasonable, see Fig. 1 in Meisel & Deibel 2017), the 31 Mg nuclei (generated by e −capture at µe = 12.2 MeV) will participate in neutron transfer reaction 28 Mg + 31 Mg → 29 Mg + 30 Mg, which leads to burnout of them on a timescale of month (for T = 5×10 8 K). As far as abundance of A = 28 nuclei is typically larger than for A = 31 nuclei (see Fig.…”

“…Fig. 2 in Meisel & Deibel 2017). Note, however, that only for A = 29 the donor for neutron transfer is the first member of URCApair, i.e.…”

“…1 (100% For applications the timescales shown in Fig. 1 should be suppressed by the abundance of acceptor nuclei, which is typically rather small ∼ 10 −3 (e.g., table 1 by Meisel & Deibel 2017). However, even after that neutron transfer can be high enough to burnout URCA pair on the accretion timescale τacc ∼ 10 2÷3 yr (e.g.…”

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

“…However, we note that we observe the burst abundances converge after ∼4 bursts. Mass fractions X(A) for species of nuclear mass A were calculated by averaging over envelope layers that no longer experienced hydrogen or helium burning, following the approach of Cyburt et al (2016); Meisel & Deibel (2017). Figure 4 and Table 1 show the ash abundance distributions corresponding to the calculated light curves of Figure 2.…”

“…There they are steadily modified by nuclear reactions, resulting in some compositional structure and driving local heat sources and heat sinks (Lau et al 2018). These modifications to the compositional and thermal structure ultimately influence the ignition of X-ray superbursts and the light curves of cooling transient neutron stars (Deibel et al 2016;Meisel & Deibel 2017). Here we focus on heat sources and sinks which have previously been identified as significant and the composition of the neutron star inner crust, whose connection to the surface burning ashes has been recently calculated using detailed reaction network calculations.…”