Polarimetry of Magnetars and Isolated Neutron Stars

Polarised radiation from isolated neutron stars provides key diagnostics on the structure of the neutron-star magnetosphere and the properties of its atmosphere. Furthermore, the detection of a large degree of polarisation is a strong evidence for the presence of vacuum birefringence, which implies that photons of different polarisations travel at different speeds through the magnetised vacuum. We outline how polarisation is generated in neutron-star atmospheres and magnetospheres, how the polarisation is preserved as the radiation travels to us, what the current observations tell us and what are the prospects for future observations.