The study of the X-ray persistent emission from magnetars offers an unprecedented opportunity to gain insight into the physical processes that occur in the presence of ultra strong magnetic fields. Up to now, most of our knowledge about magnetar sources came from spectral analysis, which allowed us to test the resonant Compton scattering (RCS) scenario and to probe the structure of the star magnetosphere. Due to the huge magnetic fields, radiation emitted from the magnetar
surface is expected to be strongly polarized, and its observed polarization pattern bears the imprint of the physical mechanisms that occur on the surface of the star, RCS onto magnetospheric charges
and quantum electro-dynamics (QED) effects as photons propagate in the magnetized vacuum (the vacuum birefringence). In this work we present and discuss the X-ray polarization detection in three magnetar sources by the Imaging X-ray Polarimetry Explorer (IXPE), comparing them with
numerical predictions given by state-of-art theoretical models.