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 per cent. 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 degree of polarization of 19 per cent, 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.
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