Supernova remnants are commonly considered to produce most of the Galactic cosmicrays via diffusive shock acceleration. However, many questions about the physical
conditions at shock fronts, such as the magnetic-field morphology close to the particle
acceleration sites, remain open. Here we report the detection of a localized polarization
signal from some synchrotron X-ray emitting regions of Tycho’s supernova remnant
made by the Imaging X-ray Polarimetry Explorer. The derived polarization degree of
the X-ray synchrotron emission is 9±2% averaged over the whole remnant, and 12±2%
at the rim, higher than the 7–8% polarization value observed in the radio band. In the
west region the polarization degree is 23 ± 4%. The X-ray polarization degree in Tycho
is higher than for Cassiopeia A, suggesting a more ordered magnetic-field or a larger
maximum turbulence scale. The measured tangential polarization direction corresponds
to a radial magnetic field, and is consistent with that observed in the radio band. These
results are compatible with the expectation of turbulence produced by an anisotropic
cascade of a radial magnetic-field near the shock, where we derive a magnetic-field
amplification factor of 3.4 ± 0.3. The fact that this value is significantly smaller than
those expected from acceleration models is indicative of highly anisotropic magneticfield turbulence, or that the emitting electrons either favor regions of lower turbulence,
or accumulate close to where the magnetic-field orientation is preferentially radially
oriented due to hydrodynamical instabilities
