Michael Wiescher

The presence of a strong shallow heat source of unknown origin in accreting neutron star crusts has been inferred by analyzing x-ray observations of their cooling in quiescence. We model the cooling of KS 1731-260 using realistic crust compositions and nuclear heating and cooling sources from detailed nuclear reaction network calculations. We find that the required strength of the shallow heat source in KS 1731-260 is reduced by more than a factor of 3 compared to previous analysis, a 5σ difference that alleviates the need for exotic solutions. Our analysis also suggests the existence of an impure nuclear pasta layer in the inner crust of KS 1731-260, though future observations will provide more stringent constraints. In addition, we obtain constraints on the dominant surface burning modes of KS 1731-260 over its history.

Here we report on the direct measurement of the resonance strengths of the E R lab = 647 keV and 1842 keV resonances in the Ca 40 ( p , γ ) Sc 41 reaction. At novae temperatures, 0.2 < T 9 < 0.7 , the Ca 40 ( p , γ ) Sc 41 reaction is governed by the low energy resonance at E R lab = 647 keV , whereas the E R lab = 1842 keV resonance serves as a normalization standard for nuclear reaction experiments within the astrophysically relevant energy range. For the E R lab = 647 keV resonance, we obtain a resonance strength ω γ = ( 2.51 ± 0 . 09 stat ± 0 . 22 syst ) meV , with an uncertainty a factor of 2.5 smaller than the previous direct measurement value. For the E R lab = 1842 keV resonance, we obtain a resonance strength ω γ = ( 0.148 ± 0 . 006 stat ± 0 . 013 syst ) eV , which is consistent with previous studies but deviates by 2 σ from the most recent measurement. Our results suggest Ca 40 to be a strong waiting point in the nucleosynthesis path of oxygen-neon (ONe) novae.