Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX\, J1856.5$-$3754

Understand

The "Magnificent Seven" (M7) are a group of radio-quiet Isolated Neutron Stars (INSs) discovered in the soft X-rays through their purely thermal surface emission.

• Owing to the large inferred magnetic fields ($B\approx 10^{13}$ G), radiation from these sources is expected to be substantially polarised, independently on the mechanism actually responsible for the thermal emission.
• A large observed polarisation degree is, however, expected only if quantum-electrodynamics (QED) polarisation effects are present in the magnetised vacuum around the star.
• The detection of a strongly linearly polarised signal would therefore provide the first observational evidence of QED effects in the strong-field regime.