Using data from a NASA satellite, the Imaging X-ray Polarimetry Explorer (IXPE), astronomers spotted a star that had a solid surface with no atmosphere.

The study- an international collaboration co-led by UCL scientists- reported a signature in the X-ray light emitted by a highly magnetized dead star called a magnetar. The team looked at IXPE’s observation of magnetar 4U 0142+61. It is located almost 13,000 light years away from Earth in the Cassiopeia constellation.

This was the first time polarised X-ray light from a magnetar had been observed.

While looking through the data, the team identified a much lower proportion of polarised light than expected if the X-rays passed through an atmosphere. The team also discovered that for light particles with higher energies, the angle of polarisation, or the “wiggle,” flipped by exactly 90 degrees in comparison to light with lower energies, as predicted by theoretical models for stars with solid crusts encircled by magnetospheres that are filled with electric currents.

Co-lead author Professor Silvia Zane (UCL Mullard Space Science Laboratory), a member of the IXPE science team, said: “This was completely unexpected. I was convinced there would be an atmosphere. The star’s gas has reached a tipping point and become solid in a similar way that water might turn to ice. This is a result of the star’s incredibly strong magnetic field.”

“But, like with water, the temperature is also a factor – a hotter gas will require a stronger magnetic field to become solid.”

“A next step is to observe hotter neutron stars with a similar magnetic field, to investigate how the interplay between temperature and magnetic field affects the properties of the star’s surface.”

Lead author Dr. Roberto Taverna, from the University of Padova, said: “The most exciting feature we could observe is the change in polarisation direction with energy, with the polarisation angle swinging by exactly 90 degrees.”

“This agrees with what theoretical models predict and confirms that magnetars are indeed endowed with ultra-strong magnetic fields.”

According to quantum theory, a strongly magnetized environment causes light to be polarised in two directions: parallel to the magnetic field and perpendicular to it. The amount and direction of the observed polarisation provide information that would not otherwise be available, leaving a trace of the magnetic field structure and the physical state of materials in the region of the neutron star.

At high energies, photons polarised perpendicularly to the magnetic field are expected to dominate, resulting in the observed 90-degree polarisation swing.

Professor Roberto Turolla, from the University of Padova, who is also an honorary professor at the UCL Mullard Space Science Laboratory, said: “The polarisation at low energies is telling us that the magnetic field is likely so strong to turn the atmosphere around the star into a solid or a liquid, a phenomenon known as magnetic condensation.”

“The solid crust of the star is thought to be composed of a lattice of ions, held together by the magnetic field. The atoms would not be spherical but elongated in the direction of the magnetic field.”

“It is still a subject of debate whether or not magnetars and other neutron stars have atmospheres. However, the new paper is the first observation of a neutron star where a solid crust is a reliable explanation.”

Professor Jeremy Heyl of the University of British Columbia (UBC) added: “It is also worth noting that including quantum electrodynamics effects, as we did in our theoretical modeling, gives results compatible with the IXPE observation. Nevertheless, we are also investigating alternative models to explain the IXPE data, for which proper numerical simulations are still lacking.”

Journal Reference:

  1. Roberto Taverna et al. Polarized x-rays from a magnetar. Science 3 Nov 2022. DOI: 10.1126/science.add0080