Develop models of phase-dependent polarization degree

Develop physical models that explain the phase dependence of the polarization degree in strongly magnetized accreting neutron stars.

Background

IXPE observations generally find much lower polarization degrees than predicted by earlier accretion-column calculations. Phase-resolved measurements show complicated behavior that may result from mixtures of polarized and unpolarized components, depolarization, reflection, vacuum polarization, and complex magnetized radiative transfer; existing models do not yet provide an adequate explanation.

References

In general, the PD shows a very complicated behavior with phase which, given the lack of models that could be used to describe its behavior, is not yet understood.

— Pulse profiles of accreting neutron stars - A review of analysis methods, observations, and theoretical models  (2608.20876 - Pottschmidt et al., 21 Aug 2026) in Section 3.2, “Polarization in pulse profiles”

Nevertheless, because the warm blackbody polarization direction derived from the spectro-polarimetric fitting appears misaligned relative to the hot component, and given that the hot blackbody already provides the larger contribution at the lower end of the IXPE\ band ($\approx2\,\mathrm{keV}$), an O-mode contribution from the condensed surface cannot be ruled out, at least at specific rotational phases (where the viewing geometry may be more favorable).

— IXPE observation of the new magnetar source EP J223759.5+531421  (2609.25958 - Taverna et al., 22 Sep 2026) in Section 6, discussion of the warm and hot thermal components