X-ray Polarization from Magnetar 1E 1547.0−5408 Supports Long-Predicted Vacuum Birefringence

A peer-reviewed study in Nature reports unusually strong X-ray polarization from the magnetar 1E 1547.0−5408, evidence that the authors say is best explained by vacuum birefringence, a long-predicted effect of quantum electrodynamics in extremely strong magnetic fields.

If that interpretation holds up, it would mark a rare observational test of how light behaves in fields far beyond anything achievable in laboratories on Earth.

The paper, “Vacuum birefringence and the polarized X-ray emission from a radio magnetar,” was published by Nature on Aug. 5 as the version of record. It lists Rachael E. Stewart and Hoa Dinh Thi among its lead and corresponding authors. The study was received Sept. 19, 2025, accepted June 29, 2026, and assigned DOI 10.1038/s41586-026-10859-z.

The target, 1E 1547.0−5408, is a radio-emitting magnetar — a highly magnetized neutron star — with a surface magnetic field of about 2.2 × 10^14 gauss and a spin period of about 2.09 seconds, according to the paper. Magnetars are among the few known natural environments extreme enough to make vacuum birefringence observable.

To study it, researchers combined data from NASA’s Imaging X-ray Polarimetry Explorer, or IXPE, NICER on the International Space Station, and the Parkes/Murriyang radio telescope in Australia. IXPE observed the source from March 26 to April 5, 2025, collecting 499.5 kiloseconds of good exposure time, or about 139 hours.

The headline result was an energy-integrated X-ray polarization degree of 46% plus or minus 4% in the 2 to 8 kilo-electron-volt band. The signal was stronger at lower energies: 59% plus or minus 5% in the 2 to 3 keV band, dropping to 37% plus or minus 5% in the 3 to 4 keV band.

In some parts of the star’s rotation, the polarization climbed even higher. The paper reports phase-resolved values reaching 74% plus or minus 11% in the 2 to 4 keV band and 82% plus or minus 15% in the 2 to 3 keV band. Overall, the 2 to 8 keV polarization detection was about 10.5 sigma above IXPE’s 99%-confidence minimum detectable polarization threshold.

The team also found that the X-ray polarization angle changes with rotation in a pattern consistent with the rotating vector model, a standard geometric framework used to interpret pulsar and magnetar polarization. The X-ray behavior also broadly tracked the source’s radio polarization geometry, giving the researchers an independent check on how the system is oriented.

That combination is central to the paper’s interpretation. The authors argue that the high polarization, its sharp decline over just a few keV, and the way it changes with rotational phase are hard to reproduce with standard surface-emission models that let light propagate without refractive effects. By contrast, they write, those features are naturally explained if magnetospheric propagation is governed by vacuum birefringence.

The claim, however, is not framed as uncontested final proof. The interpretation depends on assumptions about viewing geometry, plasma conditions and how the magnetosphere is modeled. The result is better described as strong, peer-reviewed evidence consistent with vacuum birefringence, and among the clearest X-ray tests yet.

Vacuum birefringence comes from the Heisenberg-Euler framework developed in the 1930s. In essence, quantum electrodynamics predicts that in sufficiently intense magnetic fields, even empty space should act like a medium, bending different light polarizations differently. Earlier astrophysical evidence existed at optical wavelengths, including a 2017 polarimetry result for neutron star RX J1856.5−3754 that was interpreted as consistent with the effect. The new work stands out because it probes the phenomenon in X-rays, with phase-resolved measurements and simultaneous radio constraints.

That makes radio magnetars especially valuable targets. Because they emit radio pulses as well as X-rays, astronomers can use radio polarization to better pin down the system’s geometry. As Marcus E. Lower said in a CSIRO release, “Detecting vacuum birefringence requires a magnetic field that is over 100 million times stronger than any we’ve ever made on Earth. Thankfully, nature has provided us with magnetars, which are the perfect cosmic laboratories to go looking for this effect.”