Superdense neutron star likely has solid crust, NASA telescope finds

An artist's impression of a magnetar, which is a type of neutron star.
An artist's impression of a magnetar, which is a type of neutron star. (Image credit: ESO/L. Calçada)

The first-ever X-ray observations of a magnetar, a type of neutron star with the most powerful magnetic fields in the universe, have confirmed that these fields are polarized and indicate a solid surface with no atmosphere.

Scientists used NASA's Imaging X-ray Polarimetry Explorer (IXPE) space telescope to examine the magnetar 4U 0142+61, which is located about 13,000 light-years from Earth in the constellation of Cassiopeia. 

The X-ray observations represent the first time scientists have been able to confirm the theory that magnetic fields of magnetars are polarized. That conclusion also revealed the solid and bare nature of the neutron star's surface. 

The findings could help scientists better understand the physics of extreme cosmic objects, including neutron stars and black holes

Related: See the 1st image from NASA's IXPE space telescope

The location of the magnetar 4U 0142+61, which lies about 13,000 light-years from Earth.

The location of the magnetar 4U 0142+61, which lies about 13,000 light-years from Earth. (Image credit: Roberto Taverna)

Measuring polarization tells scientists about the electric and magnetic fields that comprise wavelengths of light and which oscillate at right angles to the direction the wave is traveling in.

Light is described as being polarized when its electric fields are oriented so they oscillate in a 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," Roberto Taverna, an astronomer at the University of Padova in Italy and lead author of a study announcing the new IXPE results, said in a statement (opens in new tab).

Taverna and his colleagues found to their surprise that polarization can depend on the energy of the individual particles of light, the photons.

"Based on current theories for the magnetars, we expected to detect polarization," Martin Weisskopf, the NASA emeritus scientist who led the IXPE team from the mission's beginning until spring 2022, said in the same statement. "But no one predicted polarization would depend on energy, as we are seeing in this magnetar."

Like all neutron stars, magnetars form when massive stars run out of fuel for nuclear fusion and their cores can no longer support themselves against gravitational collapse.

This results in a body with the mass of the sun or greater compressing down to around 10 miles (16 kilometers) across, the size of an average city on Earth. As a result, the material that comprises neutron stars is so dense that NASA says a single teaspoon of it would weigh 4 billion tons.

The polarization at low energies seen by IXPE, which launched to Earth orbit atop a SpaceX Falcon 9 rocket in December 2021, indicates that the magnetic field of this magnetar is so unimaginably powerful that it could have turned the atmosphere around the neutron star into a solid or a liquid, in a phenomenon called magnetic condensation.

That depends on if magnetars and other neutron stars actually possess an atmosphere, however, something that is still hotly debated by astronomers. This mystery could be just one of those solved by studying polarized X-rays from these extreme objects with IXPE, a practice known as polarimetry, especially if further results indicate a bare surface.

"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," Weisskopf said. "Future missions will have to be cognizant of this fact."

The study (opens in new tab) reporting the results was published last month in the journal Science.

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Robert Lea
Contributing Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.