IXPE observations strengthen evidence for vacuum birefringence around magnetar 1E 1547.0−5408

Scientists using NASA’s Imaging X-ray Polarimetry Explorer, or IXPE, and coordinated radio and X-ray observations say they have found some of the strongest evidence yet for vacuum birefringence, a nearly 90-year-old prediction that empty space itself should alter light in extremely strong magnetic fields. NASA said Wednesday that the findings were published in Nature, with a public manuscript detailing the analysis also posted on arXiv.

The claim is notable because vacuum birefringence, predicted in 1936 from quantum electrodynamics, is one of the stranger consequences of modern physics: In sufficiently intense electromagnetic fields, the vacuum is not expected to behave as truly empty, but more like a medium that affects different polarizations of light in different ways. Magnetars — highly magnetized neutron stars with fields around 10^14 to 10^15 gauss — are among the few natural places where scientists can test that idea, because those fields exceed the so-called critical QED threshold of about 4.4 × 10^13 gauss.

The target in the new study was the magnetar 1E 1547.0−5408, which spins once about every 2.1 seconds. The observing campaign ran from March 26 to April 5, 2025, combining more than 140 hours of IXPE data with measurements from NASA’s NICER X-ray instrument and the Parkes/Murriyang radio telescope operated by Australia’s national science agency, CSIRO.

IXPE, NASA’s first dedicated X-ray polarimetry mission, measures the polarization of X-rays in the 2-8 keV band. In the new observations, it detected very large, phase-dependent linear polarization from the magnetar. According to the manuscript, the energy-integrated polarization degree across 2-8 keV was about 46% plus or minus 4%, with a detection significance of about 10.5 sigma. The polarization grew stronger at lower X-ray energies, reaching about 65% at 2 keV. In the 2-3 keV band, it peaked at nearly 80% during some parts of the star’s rotation, with the manuscript citing about 82% at the highest point.

The authors argue that those measurements are best explained if the X-rays propagated through a magnetar environment shaped by vacuum birefringence. Their main point is not simply that the polarization was high, but that its changes with energy and with the star’s rotation lined up with radio polarization-angle behavior measured at the same time. That combined X-ray and radio picture, they say, narrows the possible geometry of the system and makes the interpretation less ambiguous than earlier studies based on X-rays alone.

“Our model suggests that reproducing the observed X-ray polarization signatures, while also satisfying the constraints set by radio observations, requires the presence of vacuum birefringence in the neutron star’s environment,” Hoa Dinh Thi said in a NASA statement.

The study is still being presented as evidence, not final proof. Some previous analyses of IXPE magnetar data — including a 2026 Astrophysical Journal paper by Roberto Taverna and colleagues — argued that large X-ray polarization by itself is not a unique smoking gun for vacuum birefringence, because viewing geometry, plasma effects, scattering and assumptions about the star’s atmosphere can also influence the signal. This new paper argues that its multiwavelength design makes for a stronger test.

If that interpretation holds up, the result would rank among the clearest astrophysical tests yet of how the quantum vacuum behaves in extreme magnetic fields. “The information we obtained from looking at this distant star core also gives us clues about the nature of the fabric of reality as we know it, and I find that to be incredible,” Rachael Stewart said in NASA’s statement.