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Crust composition and the Shallow Heat Source in KS 1731-260

R. Jain, E. F. Brown, H. Schatz, A. V. Afanasjev, M. Beard, L. R. Gasques, J. Grace, A. Heger, G. W. Hitt, W. R. Hix, R. Lau, W. -J. Ong, M. Wiescher, Y. Xu

Abstract

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 5-sigma 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.

Crust composition and the Shallow Heat Source in KS 1731-260

Abstract

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 5-sigma 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.

Paper Structure

This paper contains 4 figures, 1 table.

Figures (4)

  • Figure 1: The abundances as a function of mass number for the ashes of superbursts (blue) Keek2012, mixed H/He bursts undergoing rp-process (red) Cyburt2016, He powered X-ray bursts (orange) Woosley2004, and stable nuclear burning (green) Schatz1999.
  • Figure 2: The upper panel shows the impurity parameter profile and the lower panel shows the nuclear heating profile in the neutron star crust. Nuclear heating is plotted as the total integrated energy deposited up to the given depth. Different curves correspond to different crust compositions considered in this study.
  • Figure 3: The best-fit cooling curves for $\mathrm{KS~{1731}{-260}}$ for different outer crust compositions without a nuclear pasta layer. The best-fit values for the shallow heating parameters are listed in Table \ref{['table:cc']}.
  • Figure 4: The same as in Fig. \ref{['fig:cc_nopasta']} but for crust compositions which include a nuclear pasta layer at the crust-core transition.