Nov. 7 () –
A signature in the X-ray light emitted by a highly magnetized dead star known as a magnetar suggests that the star it has a solid surface with no atmosphere.
An international collaborative study published in the journal Science used data from a NASA satellite, the Imaging X-ray Polarimetry Explorer (IXPE), which was released last December. The satellite, a collaboration between NASA and the Italian Space Agency, offers a new way to observe X-ray light in space by measuring its polarization, the direction of motion of light waves.
The team analyzed IXPE’s observation of magnetar 4U 0142+61, located in the constellation Cassiopeia, about 13,000 light-years from Earth. This was the first time polarized X-ray light from a magnetar had been observed.
Magnetars are neutron stars: very dense remnant cores of massive stars that have exploded as supernovae at the end of their lives. Unlike other neutron stars, they have an immense magnetic field, the most powerful in the universe.
They emit bright X-rays and show erratic periods of activity, emitting bursts and flares that can release millions of times more energy in just one second than our Sun emits in a year. They are believed to be powered by their ultra-strong magnetic fields, 100 to 1,000 times stronger than standard neutron stars.
The research team found a much smaller proportion of polarized light than would be expected if X-rays passed through an atmosphere. Polarized light is light in which all motion is moving in the same direction; that is, electric fields vibrate only one way. An atmosphere acts as a filter, selecting only one state of polarization of light.
The team also found that for light particles at higher energies, the polarization angle was reversed by exactly 90 degrees compared to light at lower energies, following what theoretical models would predict if the star had a solid crust surrounded. by an external magnetosphere filled with electrical currents.
Co-lead author, Professor at the Mullard Space Science Laboratory at UCL (University College London), a member of the IXPE science team, said it’s a statement: “This was completely unexpected. I was convinced there would be an atmosphere. The star’s gas has reached a tipping point and has turned solid in a similar way that water might turn to ice. This is the result of the star’s incredibly strong magnetic field.
“But, as with water, temperature is also a factor: hotter gas will require a stronger magnetic field to become solid. The next step is to observe hotter neutron stars with a similar magnetic field, to investigate how the interaction between temperature and magnetic field affects the properties of the star’s surface.”
Lead author Dr Roberto Taverna, from the University of Padua, said: “The most exciting feature we were able to observe is the change in polarization direction with energy, with the polarization angle swinging exactly 90 degrees.
“This is in agreement with what is predicted by theoretical models and confirms that magnetars are endowed with ultra-strong magnetic fields.”
Quantum theory predicts that light propagating in a strongly magnetized environment is polarized in two directions, parallel and perpendicular to the magnetic field. The amount and direction of polarization observed are imprinted by the structure of the magnetic field and the physical state of matter in the neighborhood of the neutron star, providing information otherwise inaccessible.
At high energies, photons (light particles) polarized perpendicular to the magnetic field are expected to dominate, resulting in the observed 90-degree polarization swing.
Professor Roberto Turolla, from the University of Padua, who is also an honorary professor at UCL’s Mullard Space Science Laboratory, said: “The polarization at low energies tells us that the magnetic field is probably so strong that it rotates the atmosphere around the star in a solid or a liquid, a phenomenon known as magnetic condensation.”
The star’s solid crust is thought to be made up 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.
Whether or not magnetars and other neutron stars have atmospheres is still a matter of debate. However, the new paper is the first observation of a neutron star. where a solid crust is a reliable explanation.
Professor Jeremy Heyl from the University of British Columbia (UBC) added: “It is also worth noting that including quantum electrodynamic effects, as we did in our theoretical model, gives results consistent with the IXPE observation. However, we are also investigating alternative models to explain the IXPE data, for which adequate numerical simulations are still lacking”.
