On the magnetic fields of Be/X-ray pulsars in the Small Magellanic Cloud

TL;DR

This paper proposes a magnetic levitation accretion model to explain the properties of Be/X-ray pulsars in the Small Magellanic Cloud, linking their spin periods to the magnetic fields of their stellar winds and companions.

Contribution

It introduces a magnetically levitating disc model for neutron star accretion, explaining pulsar periods without requiring unusually strong magnetic fields.

Findings

Pulsar periods depend on the magnetization of stellar winds.

The model accounts for the observed period scatter among pulsars.

Magnetic fields of companions influence neutron star spin equilibrium.

Abstract

We explore the possibility to explain the properties of the Be/X-ray pulsars observed in the Small Magellanic Cloud within the magnetic levitation accretion scenario. This implies that their X-ray emission is powered by a wind-fed accretion onto a neutron star (NS) which captures matter from a magnetized stellar wind. The NS in this case is accreting matter from a non-keplerian magnetically levitating disc (ML-disc) which is surrounding its magnetosphere. This allows us to explain the observed periods of the pulsars in terms of spin equilibrium without the need of invoking dipole magnetic fields outside the usual range ~ 10^11- 10^13 G inferred from cyclotron features of Galactic high mass X-ray binaries. We find that the equilibrium period of a NS, under certain conditions, depends strongly on the magnetization of the stellar wind of its massive companion and, correspondingly, on the magnetic field of the massive companion itself. This may help to explain why similar NSs in binaries with similar properties rotate with different periods yielding a large scatter of periods of the accretion-powered pulsar observed in SMC and our galaxy.