paper

Evidence for vacuum birefringence from the first optical polarimetry measurement of the isolated neutron star RX\, J1856.53754

arXiv:1610.08323 · doi:10.1093/mnras/stw2798

Abstract

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 ( 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. While polarisation measurements in the soft X-rays are not feasible yet, optical polarisation measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes --. Here, we report on the measurement of optical linear polarisation for the prototype, and brightest member, of the class, RX\, J1856.53754 (), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a polarisation degree and a polarisation position angle $\mathrm{P.A.}=145\fdg39\pm9\fdg44$, computed east of the North Celestial Meridian. The that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED.

9 pages, 7 figures, accepted for publication on MNRAS

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