Outer crust of a cold non-accreting magnetar

TL;DR

This paper models the outer crust of a cold, non-accreting magnetar, revealing how strong magnetic fields influence nuclear composition and structure, leading to the prevalence of heavier nuclei at various pressures.

Contribution

It introduces a self-consistent covariant density functional approach to study magnetic field effects on crust composition in magnetars, including nuclear mass modifications and electron Landau quantization.

Findings

Magnetic fields alter crust composition, favoring heavier nuclei.

At B > 10^{16} G, the crust predominantly consists of Zr-92.

Magnetic effects extend the pressure range of crustal matter.

Abstract

The outer crust structure and composition of a cold, non-accreting magnetar is studied. We model the outer crust to be made of fully equilibrated matter where ionized nuclei form a Coulomb crystal embedded in an electron gas. The main effects of the strong magnetic field are those of quantizing the electron motion in Landau levels and of modifying the nuclear single particle levels producing, on average, an increased binding of nucleons in nuclei present in the Coulomb lattice. The effect of an homogeneous and constant magnetic field on nuclear masses has been predicted by using a covariant density functional, in which induced currents and axial deformation due to the presence of a magnetic field that breaks time-reversal symmetry have been included self-consistently in the nucleon and meson equations of motion. Although not yet observed, for $B\gtrsim 10^{16}$G both effects contribute to produce different compositions and to enlarge the range of pressures typically present in common neutron stars. Specifically, in such a regime, the magnetic field effects on nuclei favor the appearance of heavier nuclei at low pressures. As $B$ increases, such heavier nuclei are also preferred up to larger pressures. In the most extreme case, the whole outer crust is almost made of ${}_{40}^{92}$Zr$_{52}$.