Impact of Pycnonuclear Fusion Uncertainties on the Cooling of Accreting Neutron Star Crusts

TL;DR

This study investigates how uncertainties in pycnonuclear fusion reaction rates affect the thermal evolution and cooling curves of accreting neutron star crusts, highlighting their significant impact within observational uncertainties.

Contribution

First sensitivity analysis of pycnonuclear fusion rates in neutron star crust models, coupling reaction networks with thermal evolution to assess their impact on cooling curves.

Findings

Reaction rate uncertainties shift heat deposition depth.

Enhanced rates lead to shallower heat deposition.

Variations cause up to 9 eV difference in cooling curves.

Abstract

The observation of X-rays during quiescence from transiently accreting neutron stars provides unique clues about the nature of dense matter. This, however, requires extensive modeling of the crusts and matching the results to observations. The pycnonuclear fusion reaction rates implemented in these models are theoretically calculated by extending phenomenological expressions and have large uncertainties spanning many orders of magnitude. We present the first sensitivity studies of these pycnonuclear fusion reactions in realistic network calculations. We also couple the reaction network with the thermal evolution code dStar to further study their impact on the neutron star cooling curves in quiescence. Varying the pycnonuclear fusion reaction rates alters the depth at which nuclear heat is deposited although the total heating remains constant. The enhancement of the pycnonuclear fusion reaction rates leads to an overall shallower deposition of nuclear heat. The impurity factors are also altered depending on the type of ashes deposited on the crust. These total changes correspond to a variation of up to 9 eV in the modeled cooling curves. While this is not sufficient to explain the shallow heat source, it is comparable to the observational uncertainties and can still be important for modeling the neutron star crust.