Monday

The polarisation properties of the thermal emission from a neutron star (NS) depends
on both the state of the matter at the star surface and the properties of the vacuum outside the star. In particular, the magnetic field present in strongly magnetised NSs, B>1e+13 G, may drive a phase transition turning a gaseous atmosphere into to a condensed metallic surface and induce QED vacuum birefringence in the star magnetosphere. In this talk, I will present our calculations of phase-averaged polarisation fraction and polarisation angle for X-ray dim isolated NSs (B~1e+13 G) and transient magnetars (B~1e+14 G). I will show how polarimetry observations in the optical and X-ray band can give us insight about the properties of the matter and the vacuum under strong magnetic fields. I will also discuss the recent optical polarimetry observation of RX J1856.5-3754 and the expectations for X-ray polarimetry with future missions such as XIPE and eXTP.