A magnetar located in our Galaxy could provide one of the clearest pieces of evidence for a quantum effect predicted since the 1930s. By observing the polarization of its X-rays, a team found behavior that is difficult to reproduce without vacuum birefringence. This phenomenon would make the vacuum itself react to an extreme magnetic field.
A magnetar is a neutron star with a gigantic magnetic field. The studied object, 1E 1547.0-5408, also emits radio waves. Its surface field exceeds 1010 teslas, intensities impossible to produce sustainably in a laboratory.
Magnetars are among the most exotic objects in the Universe.
Credit: ESA
At such values, quantum electrodynamics predicts that the vacuum no longer behaves exactly as an empty, passive space. Quantum fluctuations slightly modify the propagation of light. Depending on its polarization orientation, light then does not cross the magnetic field in the same way: this is vacuum birefringence.
To search for this signature, the researchers used the IXPE satellite, specialized in X-ray polarization. The observations were complemented by the NICER space telescope and by radio measurements from the Parkes radio telescope in Australia. This combination helps constrain the star’s geometry and the orientation of its field.
Without vacuum birefringence, the models fail to coherently reproduce all the measurements and geometric constraints. The expected effect acts as if the vacuum imposed two slightly different optical paths depending on polarization. On the scale of a magnetar, this small difference can strongly modify the signal ultimately received.
Caution is nevertheless necessary. This is not a laboratory experiment in which a single parameter can be isolated at will. The conclusions also depend on the atmosphere and magnetosphere models used to interpret the data. Future polarimetric observations, on this magnetar and on other neutron stars, will need to verify whether the same signature reappears in different configurations.