  X-ray polarimetry, based on Gas Pixel Detectors (GPDs), have reached a high
level of maturity with the Imaging X-ray Polarimeter Explorer (IXPE) leading to
the first ever spatially resolved polarimetric measures. However, being this a
new technique, a few unexpected effect have emerged during in flight
operations. In particular it was almost immediately found that on-board
unpolarized calibration sources were showing radially polarized halos. The
origin of this features was recognized in a correlation between the error in
reconstructing the absorption point of the X-ray photon and the direction of
its electric field vector. Here we present and discuss in detail this effect,
showing that it is possible to provide a simple and robust mathematical
formalism to handle it. We further show its role and relevance for the recent
IXPE measures, and for the use of GPD-based techniques in general, and
illustrate how to model it in the study of extended sources.

Instrumentation and Methods for Astrophysics;High Energy Astrophysical Phenomena;High Energy Physics - Experiment