On the Evolution of Binary Neutron Stars

TL;DR

This paper models the magnetic, spin, and orbital evolution of binary neutron stars, revealing that millisecond pulsars can form despite magnetic field increases, with evolution heavily influenced by initial conditions and binary parameters.

Contribution

It introduces a novel model for magnetic field evolution in binary neutron stars, highlighting differences from single stars and implications for millisecond pulsar formation.

Findings

Binary neutron stars' core magnetic field can increase during accretion.

Millisecond pulsars can form despite magnetic field growth.

Evolution depends on initial orbital period and neutron star properties.

Abstract

In this paper we investigate the evolution of binary neutron stars, namely, their magnetic field, spin, and orbital evolution. The core of a neutron star is considered to be a superfluid, superconductor type II. Flux expulsion of the magnetic field out of the core of a single neutron star has been discussed by previous authors. However, the evolution of the core magnetic field is substantially different for a binary neutron star. While for a single neutron star the fluxoids of the proton superconductor always move outward through the core, in a binary neutron star in the accretion phase fluxoids move back into the core. The subsequent increase of the core magnetic field results in the increase of the surface magnetic field. We ask weather within the framework of this model the formation of millisecond pulsars (MSPs) is possible. We show that despite the increase of the core magnetic field, MSPs are formed in this model. The evolution of the neutron star with various orbital periods, magnetic fields, spin periods, and other parameters are numerically investigated. The equation of state of the neutron star, initial orbital period of the binary, and other parameters of the binary have substantial effects on the evolution of period vs. magnetic field.