Light from a distant star is not just a steady stream. It oscillates like a wave and can vibrate in different directions. Usually these oscillations are indistinguishable, but a magnetar can sort them out. Its field is so dense that it acts like a polarizing filter: one part of the wave slips through a bit faster, while the other gets stuck, as if running knee-deep in water. As a result, light reaches us with a tiny delay of one half relative to the other.
Astrophysicists took a realistic model of a magnetar's magnetic field and calculated how strongly this effect would manifest. It turns out the delay could be ten times larger than old estimates — previously it was considered almost undetectable. Then they calculated what the light from all known magnetars would look like and compared it with the sensitivity of two X-ray telescopes: IXPE and eXTP. Both instruments can notice the difference, but one object stands out particularly clearly.
Such an experiment is not just a test of theory. If the delay is confirmed, scientists will be able to measure the strength of the magnetic field near the star directly, without intermediaries. It is also a window into quantum electrodynamics: a strong field turns emptiness into a sort of lens, and light behaves as if the space around the star has become an anisotropic crystal. The magnetar ceases to be just a bright dot and becomes a laboratory where the laws of physics are tested to their limits.
A magnetar's magnetic field is billions of times stronger than that of a medical MRI scanner. If such an object were in place of the Moon, it would wipe the data from all credit cards on Earth.
In Peter F. Hamilton's novel 'Pandora's Star', ships use polarized radiation from neutron stars for navigation in deep space — much like described in this work.