Emission from neutron stars with condensed surface: a systematic analysis

Amongst the various classes of neutron stars, magnetars are the ones hosting the strongest magnetic fields. The study of their emission in the X-ray band allows to achieve unprecedented knowledge of the physical processes in the presence of ultra-strong magnetic fields. In particular, magnetar emission is expected to be highly polarized. Thanks to the recently launched Imaging X-ray Polarimetry Explorer (IXPE), polarimetric measurements in the X-ray band are finally possible, opening an entirely new window in the investigation of magnetar physics, as well as allowing an insight into QED effects in strong magnetic fields such as vacuum birefringence. The focus of this thesis is on the emission properties of neutron stars with a condensed surface. Magnetic condensation is expected to set in for large enough fields (B > 10^13-10^14 G) and relatively low temperatures (T < 1 keV), such as those found in magnetars. In this work we present a systematic analysis of the radiation emitted from sources with a condensed surface, accounting for different geometries and physical parameters.