A dense stellar remnant with an exceptionally powerful magnetic field has given physicists their clearest observational evidence yet for an unusual property of empty space. A study published in Nature on August 5 reports polarization measurements of the magnetar 1E 1547.0-5408 that are difficult to reproduce unless a quantum effect known as vacuum birefringence is included.
Magnetars are neutron stars left behind after massive stars collapse. Their magnetic fields reach strengths that cannot be recreated in laboratories. Quantum electrodynamics predicts that, under such extreme conditions, the vacuum does not behave as completely featureless emptiness. Instead, the magnetic field can alter how light travels, with different polarization orientations propagating differently.
Researchers combined more than 140 hours of X-ray observations made by NASA's Imaging X-ray Polarimetry Explorer in March and April 2025 with data from NASA's NICER instrument and radio measurements from Australia's Murriyang, the Parkes radio telescope. The campaign let the team compare X-ray and radio polarization as the magnetar completed a rotation roughly every two seconds.
The Nature paper reports phase-averaged X-ray polarization of about 65 percent at an energy of 2 kiloelectronvolts. During some portions of the rotation, polarization in the 2-to-3 kiloelectronvolt band approached 80 percent. Conventional surface-emission models without vacuum birefringence did not adequately explain that combination of high polarization and smooth rotational changes.
Computer simulations showed that propagation controlled by vacuum birefringence could naturally produce the observed signal. That makes the result strong support for the quantum prediction, but it is not the same as a final laboratory-style proof. The researchers and NASA both frame it as a possible first direct observation and say additional measurements will be needed.
The work demonstrates how compact stars can test physical regimes unavailable on Earth. It may also help scientists separate effects created near a neutron star's surface from those produced as radiation travels through its magnetosphere.