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

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 (B ≈ 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 quantumelectrodynamics (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 ≈ 26-28. Here, we report on the measurement of optical linear polarisation for the prototype, and brightest member, of the class, RX J1856.5−3754 (V ∼ 25.5), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a polarisation degree P.D. = 16.43% ± 5.26% and a polarisation position angle P.A. = 145. • 39 ± 9. • 44, computed east of the North Celestial Meridian. The P.D. that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED.

An addendum to the Heisenberg-Euler effective action beyond one loop

“…Recently, indications have been reported for the relevance of QED vacuum birefringence for optical polarimetry of a neutron star [36]. Other theoretical proposals for optical signatures of quantum vacuum nonlinearity have focused on photon-photon scattering in the form of laser-pulse collisions [37][38][39][40], interference effects [41][42][43][44], quantum reflection [45,46], photon merging [47][48][49][50][51], photon splitting [19,[52][53][54][55][56][57][58][59][60], and higher-harmonic generation from laser driven vacuum [61][62][63][64].…”

A note on polarized light from magnetars

“…5 in [1]. However, in what follows, we want to approach quantitatively this analysis, relying only on the "isotropic blackbody" model as presented in [1] and further discussed in [23]. The experimental result is a "3-σ " one, meaning that, at ≈ 99% CL, the polarization degree is larger than zero.…”

“…In principle a 100% linear polarization should be measured. 1 If we take π R NS (with R NS = 10 km) the angle between two B vectors at θ = π/4, on a sphere with radius R NS , is ψ = 90 • whereas on a sphere of R = 50 km is ψ 24 • and ψ 12 • at 100 km. Since polarizations follow the magnetic field vectors, as discussed above, a very significant enhancement of the polarization effect is expected to occur even on a length scale of 50 km.…”

“…The derivative of the phase factor S(z) in e i S(z) is extremely large ∼ 10 6 when z is of the order of the radius of the star and rapidly decreases at about 50 km from the star center. We take ω ≈ 1 eV since measurements in [1] are done for visible light. Thus we find that within these distances, the rapidly oscillating function e i S(z) makes |E ⊥ (z)| 2 negligible: if a polarization mode is not initially present on the surface of the star it will not be produced along the distance the light travels in getting far out from the star surface.…”

“…Even if our calculations, briefly summarized in Fig. 2, are capturing the essential features of the phenomenon, in the next section we will compare the theoretical models, including all the mentioned effects, with the experimental data reported in [1].…”