  Supernova remnants are commonly considered to produce most of the Galactic
cosmic rays 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
magnetic-field 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.
