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

Abstract

We report on ixpe observations of the Be-transient X-ray pulsar LS V +44 17/RX J0440.9+4431 at two luminosity levels during the giant outburst in January--February 2023. Considering the observed spectral variability and changes in the pulse profiles, the source was likely caught in super- and sub-critical states with significantly different emission region geometry, associated with the presence of accretion columns and hot spots, respectively. We focus here on the pulse-phase resolved polarimetric analysis and find that the observed dependencies of the polarization degree and polarization angle (PA) on pulse phase are indeed drastically different for the two observations. The observed differences, if interpreted within the framework of the rotating vector model (RVM), imply dramatic variations of the spin axis inclination and the position angle and the magnetic colatitude by tens of degrees within just a few days separating the observations. We suggest that the apparent changes in the observed PA phase dependence are predominantly related to the presence of a polarized unpulsed component in addition to the polarized radiation associated with the pulsar itself. We show that the observed PA phase dependence in both observations can then be explained with a single set of RVM parameters defining the pulsar's geometry. We also suggest that the additional polarized component is likely produced by scattering of the pulsar radiation off the equatorial disk wind.