Magnetars: How Magnetic Fields Fool Light express
Neutron stars are the ultra-dense remnants of stars, where matter is compressed to the limit described by Subrahmanyan Chandrasekhar. Magnetars are a type of neutron star with a magnetic field a quadrillion times stronger than Earth's. Normally, light passing a massive body bends because spacetime is warped: it acts like a gravitational lens. But in magnetars, the field adds a second lens: the light bends even more. It's like looking through glasses with double lenses: one from gravity, one from magnetism. Without accounting for the second lens, the error in radius reaches 10%.
Future telescopes, analyzing the flicker of pulsars (discovered by Jocelyn Bell Burnell), will refine neutron star sizes. This will unlock the secrets of ultra-dense matter inaccessible on Earth. And 10% isn't just a number: for a neutron star, it's the line between stability and collapse into a black hole.
Original abstract
as written by the authors arXiv:2605.17048
Magnetars are among the most extreme laboratories in the universe, harboring surface magnetic fields reaching $$10^{15}$$~G. At these supercritical scales, Maxwell's linear electrodynamics is superseded by Nonlinear Electrodynamics (NLED). While vacuum birefringence has provided initial observational evidence for these effects, its broader impact on photon propagation remains largely unexplored. In this work, we demonstrate that NLED significantly alters photon propagation in the vicinity of magnetars, deviating light from standard null-geodesics. We estimate that neglecting these corrections leads to relative errors in inferred stellar radii by means of ray-tracing techniques of approximately $$10\%$$. Furthermore, we find that NLED induces a systematic minimal travel-time delay of approximately $$350~n$$s, a value that already far exceeds the $$100$$~ns temporal resolution of missions like NICER. These results are critical for the interpretation of X-ray pulse profiles from current and future observatories, such as eXTP, which rely on high-precision light-bending and timing models to determine neutron-star masses and radii. Finally, our results underscore the role of magnetars as a vital window into the physics of superdense matter and supercritical fields, and we briefly highlight other astrophysical observables--such as glitches and antiglitches--that may be affected by NLED.
A magnetar with a 10¹⁵ G field can demagnetize all bank cards within 100,000 km, a quarter of the way to the Moon.