  Magnetars are believed to host the strongest magnetic fields in the present
universe ($B\gtrsim10^{14}$ G) and the study of their persistent emission in
the X-ray band offers an unprecendented opportunity to gain insight into
physical processes in the presence of ultra-strong magnetic fields. Up to now,
most of our knowledge about magnetar sources came from spectral analysis, which
allowed to test the resonant Compton scattering scenario and to probe the
structure of the star magnetosphere. On the other hand, radiation emitted from
magnetar surface is expected to be strongly polarized and its observed
polarization pattern bears the imprint of both scatterings onto magnetospheric
charges and QED effects as it propagates in the magnetized vacuum around the
star. X-ray polarimeters scheduled to fly in the next years will finally allow
to exploit the wealth of information stored in the polarization observables.
Here we revisit the problem of assessing the spectro-polarimetric properties of
magnetar persistent emission. At variance with previous investigations, proper
account for more physical surface emission models is made by considering either
a condensed surface or a magnetized atmosphere. Results are used to simulate
polarimetric observations with the forthcoming Imaging X-ray Polarimetry
Explorer (IXPE). We find that X-ray polarimetry will allow to detect QED vacuum
effects for all the emission models we considered and to discriminate among
them.
