Super-Strong Magnetic Fields of Neutron Stars in Be X-Ray Binaries Estimated with New Torque and Magnetosphere Models

TL;DR

This paper revises the estimates of neutron star magnetic fields in Be X-ray binaries using improved torque and magnetosphere models, revealing significantly higher fields and more consistent solutions near spin equilibrium.

Contribution

Introduces a new torque model and applies it to different magnetosphere radius models, providing more accurate and consistent magnetic field estimates for neutron stars in Be X-ray binaries.

Findings

Magnetic field estimates are higher with the new models.

The compressed magnetosphere model yields unique solutions near spin equilibrium.

Magnetic fields range between quantum critical and virial limits.

Abstract

We re-estimate the surface magnetic fields of neutron stars (NSs) in Be X-ray binaries (BeXBs) with different models of torque, improved beyond Klus et al. (2014). In particular, a new torque model (Dai \& Li 2006) is applied to three models of magnetosphere radius. Unlike the previous models, the new torque model does not lead to divergent results for any fastness parameter. The inferred surface magnetic fields of these NSs for the two compressed magnetosphere models are much higher than that for the uncompressed magnetosphere model. The new torque model using the compressed-magnetosphere radius (Shi, Zhang \& Li 2014) leads to unique solutions near spin equilibrium in all cases, unlike other models that usually give two branches of solutions. Although our conclusions are still affected by the simplistic assumptions about the magnetosphere radius calculations, we show several groups of possible surface magnetic field values with our new models when the interaction between the magnetosphere and the infalling accretion plasma is considered. The estimated surface magnetic fields for NSs BeXBs in the Large Magellanic Cloud, the Small Magellanic Cloud and the Milk Way are between the quantum critical field and the maximum "virial" value by the spin equilibrium condition.