Are There Magnetars in High Mass X-ray Binaries? The Case of SuperGiant Fast X-Ray Transients

TL;DR

This paper investigates the role of magnetic barriers in high mass X-ray binaries, proposing that transitions between accretion regimes can explain the extreme luminosity variability observed in supergiant fast X-ray transients, potentially indicating magnetar-like fields.

Contribution

It introduces a model of wind accretion involving magnetic and centrifugal barriers, explaining large luminosity swings and suggesting the presence of magnetar-like fields in certain binaries.

Findings

Transitions across accretion regimes can cause luminosity swings of ~10^4.

Supergiant fast X-ray transients may host magnetar-like magnetic fields.

Variability can be explained by modest changes in stellar wind properties.

Abstract

In this paper we survey the theory of wind accretion in high mass X-ray binaries hosting a magnetic neutron star and a supergiant companion. We concentrate on the different types of interaction between the inflowing wind matter and the neutron star magnetosphere that are relevant when accretion of matter onto the neutron star surface is largely inhibited; these include the inhibition through the centrifugal and magnetic barriers. Expanding on earlier work, we calculate the expected luminosity for each regime and derive the conditions under which transition from one regime to another can take place. We show that very large luminosity swings (~10^4 or more on time scales as short as hours) can result from transitions across different regimes. The activity displayed by supergiant fast X-ray transients, a recently discovered class of high mass X-ray binaries in our galaxy, has often been interpreted in terms of direct accretion onto a neutron star immersed in an extremely clumpy stellar wind. We show here that the transitions across the magnetic and/or centrifugal barriers can explain the variability properties of these sources as a results of relatively modest variations in the stellar wind velocity and/or density. According to this interpretation we expect that supergiant fast X-ray transients which display very large luminosity swings and host a slowly spinning neutron star are characterized by magnetar-like fields, irrespective of whether the magnetic or the centrifugal barrier applies. Supergiant fast X-ray transients might thus provide a new opportunity to detect and study magnetars in binary systems.