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The "Magnificent Seven" (M7) are a group of radio-quiet Isolated NeutronStars (INSs) discovered in the soft X-rays through their purely thermal surfaceemission. 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. Alarge observed polarisation degree is, however, expected only ifquantum-electrodynamics (QED) polarisation effects are present in themagnetised vacuum around the star. The detection of a strongly linearlypolarised signal would therefore provide the first observational evidence ofQED effects in the strong-field regime. While polarisation measurements in thesoft X-rays are not feasible yet, optical polarisation measurements are withinreach also for quite faint targets, like the M7 which have optical counterpartswith magnitudes $\approx 26$--$28$. Here, we report on the measurement ofoptical linear polarisation for the prototype, and brightest member, of theclass, RX\, J1856.5$-$3754 ($V\sim 25.5$), the first ever for one of the M7,obtained with the Very Large Telescope. We measured a polarisation degree$\mathrm{P.D.} =16.43\% \pm5.26\%$ and a polarisation position angle$\mathrm{P.A.}=145\fdg39\pm9\fdg44$, computed east of the North CelestialMeridian. The $\mathrm{P.D.}$ that we derive is large enough to support thepresence of vacuum birefringence, as predicted by QED.

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