The high optical polarization in the Be/X-ray binary EXO 2030+375

TL;DR

This paper reports the first optical polarimetric observations of the Be/X-ray binary EXO 2030+375, revealing an unprecedented 19% polarization likely caused by magnetic grain alignment in the star's disc.

Contribution

It presents the first optical polarization measurement of EXO 2030+375, showing an exceptionally high polarization degree and proposing a novel magnetic grain alignment mechanism.

Findings

Optical polarization in EXO 2030+375 is 19%, the highest in such systems.

Interstellar medium cannot explain the high polarization, indicating an intrinsic source.

Magnetic alignment of ferromagnetic grains may cause the observed polarization.

Abstract

Polarization in classical Be stars results from Thomson scattering of the unpolarized light from the Be star in the circumstellar disc. Theory and observations agree that the maximum degree of polarization from isolated Be stars is < 4%. We report on the first optical polarimetric observations of the Be/X-ray binary EXO\,2030+375. We find that the optical (R band) light is strongly linearly polarized with a degee of polarization of 19%, the highest ever measured either in a classical or Be/X-ray binary. We argue that the interstellar medium cannot account for this high polarization degree and that a substantial amount must be intrinsic to the source. We propose that it may result from the alignment of non-spherical ferromagnetic grains in the Be star disc due to the strong neutron star magnetic field.