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

Chang-Sheng Shi, Shuang-Nan Zhang, Xiang-Dong Li

Published 2015-09-21Version 1

We re-estimate the surface magnetic fields of neutron stars 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 divergent results for any fastness parameter. The inferred surface magnetic fields of these neutron stars 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 neutron stars BeXBs in LMC, SMC and the Milk Way are between the quantum critical field and the maximum "Virial" value by the spin equilibrium condition.