The Long Quest for Vacuum Birefringence in Magnetars: 1E 1547.0–5408 and the Elusive Smoking Gun

Magnetars are now known to be among the most strongly polarized celestial sources in X-rays. Here, we report on the 500 ks observation of the magnetar 1E 1547.0-5408 performed by the Imaging X-ray Polarimetry Explorer (IXPE) in 2025 March. The IXPE spectrum is well reproduced by a single thermal component with blackbody temperature kTBB similar to 0.67 keV and emission radius RBB similar to 1.2 km. The source exhibits a high linear polarization degree in the 2-6 keV band (PD = 47.7% +/- 2.9%) with polarization angle PA=75.degrees 8 +/- 1.degrees 8 , measured west of celestial north. While PA does not appear to vary with energy, there is some evidence (at the 1 sigma confidence level) of a minimum in PD between 3 and 4 keV, compatible with what is expected by partial mode conversion at the vacuum resonance in a magnetized atmosphere. Phase-resolved spectral and polarimetric analyses reveal that X-ray thermal radiation likely originates from a single, fairly small hot spot with a nonuniform temperature distribution. Fitting the phase-dependent PA measured by IXPE with a rotating vector model (RVM) constrains the source geometry and indicates that both the dipole axis and line-of-sight are misaligned with respect to the spin axis. Under these conditions, the high polarization of the source cannot be regarded as compelling evidence for the presence of vacuum birefringence in the star's magnetosphere. Nevertheless, the fact that the RVM successfully reproduces the modulation of the X-ray polarization angle and the behavior of PD with the energy hint once more at the presence of QED effects in magnetars.