  After about 16 years since its first outburst, the transient neutron star
low-mass X-ray binary XTE J1701$-$462 turned on again in September 2022,
allowing for the first study of its X-ray polarimetric characteristics by a
dedicated observing program with the Imaging X-ray Polarimeter Explorer (IXPE).
Polarimetric studies of XTE J1701$-$462 have been expected to improve our
understanding of accreting weakly magnetized neutron stars, in particular, the
physics and the geometry of the hot inner regions close to the compact object.
The IXPE data of two triggered observations were analyzed using time-resolved
spectroscopic and polarimetric techniques, following the source along its
Z-track of the color-color diagram. During the first pointing on 2022 September
29, an average 2-8 keV polarization degree of 4.6$\pm$ 0.4\% was measured, the
highest value found up to now for this class of sources. Conversely, only a
$\sim$0.6\% average degree was obtained during the second pointing ten days
later. The polarimetric signal appears to be strictly related to the higher
energy blackbody component associated with the boundary layer (BL) emission and
its reflection from the inner accretion disk, and it is as strong as 6.1\% and
1.2\% ($>95\%$ significant) above 3-4 keV for the two measurements,
respectively. The variable polarimetric signal is apparently related to the
spectral characteristics of XTE J1701$-$462, which is the strongest when the
source was in the horizontal branch of its Z-track and the weakest in the
normal branch. These IXPE results provide new important observational
constraints on the physical models and geometry of the Z-sources. Here, we
discuss the possible reasons for the presence of strong and variable
polarization among these sources.
