Strong magnetic fields bend light in ways that bias neutron-star size measurements

Near magnetars, magnetic fields exceed the quantum electrodynamic critical threshold, causing photons to deviate from standard null geodesics in ways that linear Maxwell theory ignores. Accounting for this nonlinear electrodynamics, radius estimates from ray-tracing techniques carry roughly 10% systematic error, and light-travel times are delayed by at least 350 ns — more than three times NICER's 100 ns timing resolution. These biases propagate directly into neutron-star mass and radius inferences from X-ray pulse profiles. Missions like eXTP, designed for even higher precision, will need to incorporate these corrections to avoid drawing false conclusions about the dense-matter equation of state.