Heating in Magnetar Crusts from Electron Captures

TL;DR

This paper investigates how electron captures and fusion reactions in magnetar crusts can serve as internal heat sources, with implications for understanding their thermal luminosity and magnetic field evolution.

Contribution

It analytically and numerically examines the heat released by electron captures and fusion in magnetar crusts, linking magnetic field strength to reaction locations and providing inputs for neutron star cooling models.

Findings

Heat sources are concentrated at densities 10^{10}-10^{11} g/cm^3.

Heat power is estimated at 10^{35}-10^{36} erg/s.

Reaction locations are sensitive to magnetic field strength.

Abstract

The persistent thermal luminosity of magnetars and their outbursts suggest the existence of some internal heat sources located in their outer crust. The compression of matter accompanying the decay of the magnetic field may trigger exothermic electron captures and, possibly, pycnonuclear fusions of light elements that may have been accreted onto the surface from the fallback of supernova debris, from a disk or from the interstellar medium. This scenario bears some resemblance to deep crustal heating in accreting neutron stars, although the matter composition and the thermodynamic conditions are very different. The maximum possible amount of heat that can be released by each reaction and their locations are determined analytically taking into account the Landau--Rabi quantization of electron motion. Numerical results are also presented using experimental, as well as theoretical nuclear data. Whereas the heat deposited is mainly determined by atomic masses, the locations of the sources are found to be very sensitive to the magnetic field strength, thus providing a new way of probing the internal magnetic field of magnetars. Most sources are found to be concentrated at densities $10^{10}-10^{11}$ g cm$^{-3}$ with heat power $W^\infty\sim 10^{35}-10^{36}$ erg/s, as found empirically by comparing cooling simulations with observed thermal luminosity. The change of magnetic field required to trigger the reactions is shown to be consistent with the age of known magnetars. This suggests that electron captures and pycnonuclear fusion reactions may be a viable heating mechanism in magnetars. The present results provide consistent microscopic inputs for neutron star cooling simulations, based on the same model as that underlying the Brussels-Montreal unified equations of state.