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Magnetars are the most strongly magnetized neutron stars, and one of the mostpromising targets for X-ray polarimetric measurements. We present here thefirst Imaging X-ray Polarimetry Explorer (IXPE) observation of the magnetar1RXS J170849.0-400910, jointly analysed with a new Swift observation andarchival NICER data. The total (energy and phase integrated) emission in the2-8 keV energy range is linerarly polarized, at a ~35% level. Thephase-averaged polarization signal shows a marked increase with energy, rangingfrom ~20% at 2-3 keV up to ~80% at 6-8 keV, while the polarization angle remainconstant. This indicates that radiation is mostly polarized in a singledirection. The spectrum is well reproduced by a combination of either twothermal (blackbody) components or a blackbody and a power law. Both thepolarization degree and angle also show a variation with the spin phase, andthe former is almost anti-correlated with the source counts in the 2-8 keV and2-4 keV bands. We discuss the possible implications and interpretations, basedon a joint analysis of the spectral, polarization and pulsation properties ofthe source. A scenario in which the surface temperature is not homogeneous,with a hotter cap covered by a gaseous atmosphere and a warmer region in acondensed state, provides a satisfactory description of both the phase- andenergy-dependent spectro-polarimetric data. The (comparatively) small size ofthe two emitting regions, required to explain the observed pulsations, does notallow to reach a robust conclusion about the presence of vacuum birefringenceeffects.

A strong X-ray polarization signal from the magnetar 1RXS J170849.0-400910