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We report on Imaging X-ray polarimetry explorer (IXPE) observations of theBe-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 made at two luminositylevels during the giant outburst in January--February 2023. Considering theobserved spectral variability and changes in the pulse profiles, the source waslikely caught in supercritical and subcritical states with significantlydifferent emission-region geometry, associated with the presence of accretioncolumns and hot spots, respectively. We focus here on the pulse-phase-resolvedpolarimetric analysis and find that the observed dependencies of thepolarization degree and polarization angle (PA) on the pulse phase are indeeddrastically different for the two observations. The observed differences, ifinterpreted within the framework of the rotating vector model (RVM), implydramatic variations in the spin axis inclination, the position angle, and themagnetic colatitude by tens of degrees within the space of just a few days. Wesuggest that the apparent changes in the observed PA phase dependence arepredominantly related to the presence of an unpulsed polarized component inaddition to the polarized radiation associated with the pulsar itself. We thenshow that the observed PA phase dependence in both observations can beexplained with a single set of RVM parameters defining the pulsar's geometry.We also suggest that the additional polarized component is likely produced byscattering of the pulsar radiation in the equatorial disk wind.

Complex variations of X-ray polarization in the X-ray pulsar LS V +44 17/RX J0440.9+4431