The first X-ray observations of a type of neutron star, 4U 0142+61, suggests it has a solid crust surface and powerful, highly-polarized magnetic fields. The observations confirm theories about the magnetic fields and surfaces of magnetars.

The findings were collected through NASA’s Imaging X-ray Polarimetry Explorer (IXPE) and published in the journal Science. The data will help X-ray astronomers have a better understanding of the physics of extreme objects, such as magnetars and black holes. 

“In my mind, there can be no question that IXPE has shown that X-ray polarimetry is important and relevant to furthering our understanding of how these fascinating X-ray systems work,” said Martin Weisskopf, a NASA emeritus scientist who led the IXPE team until spring 2022. “Future missions will have to be cognizant of this fact.”

Neutron stars are the dense cores left from a star that have gone supernova and exploded. They are only 15 to 30 kilometres wide, but can weigh three times the mass of our Sun. NASA said a single teaspoon of material from a neutron star would weigh 4 billion tons. This makes neutron stars one of the densest objects in the Universe, second only to black holes.

Magnetars have the most powerful known magnetic fields in the Universe. A magnetar has a magnetic field roughly 1,000 times more powerful than a typical neutron star. This is a trillion times stronger than the Earth’s magnetic field.

Astronomers used IXPE to study the magnetar 4U 0142+61, which is 13,000 light years from Earth and located in the Cassiopeia constellation.

Measuring polarization can tell scientists about the electric and magnetic fields that make up all wavelengths of light. These fields oscillate and vibrate at right angles to the light’s path of travel. The light is polarized when the electric fields vibrate in a single and unified direction.

“We found that the angle of polarization swings by exactly 90 degrees, following what theoretical models would predict if the star had a solid crust surrounded by an external magnetosphere filled with electric currents,” said Roberto Taverna of the University of Padova and lead author of the study in a statement.

Weisskopf expected to find polarization based on current magnetar theories, but researchers were surprised to learn energy levels can affect polarization. 

It is still being debated if neutron stars could have atmospheres. But polarization at low energies indicates the magnetic field is strong enough to turn any atmosphere into a solid or liquid. This process is called magnetic condensation.

IXPE is currently being used to study a range of extreme X-ray sources.