  Magnetars are the most strongly magnetized neutron stars, and one of the most
promising targets for X-ray polarimetric measurements. We present here the
first Imaging X-ray Polarimetry Explorer (IXPE) observation of the magnetar
1RXS J170849.0-400910, jointly analysed with a new Swift observation and
archival NICER data. The total (energy and phase integrated) emission in the
2-8 keV energy range is linerarly polarized, at a ~35% level. The
phase-averaged polarization signal shows a marked increase with energy, ranging
from ~20% at 2-3 keV up to ~80% at 6-8 keV, while the polarization angle remain
constant. This indicates that radiation is mostly polarized in a single
direction. The spectrum is well reproduced by a combination of either two
thermal (blackbody) components or a blackbody and a power law. Both the
polarization degree and angle also show a variation with the spin phase, and
the former is almost anti-correlated with the source counts in the 2-8 keV and
2-4 keV bands. We discuss the possible implications and interpretations, based
on a joint analysis of the spectral, polarization and pulsation properties of
the 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 a
condensed state, provides a satisfactory description of both the phase- and
energy-dependent spectro-polarimetric data. The (comparatively) small size of
the two emitting regions, required to explain the observed pulsations, does not
allow to reach a robust conclusion about the presence of vacuum birefringence
effects.
