A signature in the X-ray light emitted by a highly magnetized dead star, known as a magnetar, suggests that the star has a solid surface with no atmosphere. The team looked at the IXPE observation of the magnetar 4U 0142+61, located in the constellation Cassiopeia, about 13,000 light-years from Earth. This was the first time that polarized X-ray light from a magnetar was observed.
Magnetars are neutron stars, very dense remnant cores of massive stars that exploded as supernovae at the end of their lives. Unlike other neutron stars, they have a huge magnetic field, the most powerful in the universe. They emit bright X-rays and show chaotic periods of activity, emitting bursts and eruptions that can release in just one second an amount of energy millions of times greater than what our Sun emits in a year. They are thought to be powered by their ultra-strong magnetic fields, 100 to 1000 times stronger than standard neutron stars. The research team found a much lower proportion of polarized light than would be expected if X-rays were passing through an atmosphere. (Polarized light is light in which the movement is in the same direction — that is, the electric fields vibrate in only one way. The atmosphere acts as a filter, selecting only one state of polarization of the light.)
The team also found that for particles of light at higher energies, the angle of polarization – the wobble – is 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 outer magnetosphere filled with electric currents. Co-author Professor Sylvia 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 has become solid in a similar way that water can turn to ice.This is a 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 interplay between temperature and magnetic field affects the properties of the star’s surface.”
Lead author Dr Roberto Taverna of the University of Padua said: “The most exciting feature we could observe is the change in polarization direction with energy, with the polarization angle swinging by exactly 90 degrees.” This is in agreement with theoretical models predict and confirm that magnetars are indeed endowed with ultra-strong magnetic fields.”
Quantum theory predicts that light propagating in a strongly magnetized medium is polarized in two directions, parallel and perpendicular to the magnetic field. The amount and direction of the observed polarization bear the imprint of the structure of the magnetic field and of the physical state of the matter near the neutron star, providing information unavailable otherwise. At high energies, photons (particles of light) polarized perpendicular to the magnetic field are expected to dominate, leading to the observed 90-degree polarization.
Professor Roberto Turola of the University of Padua, who is also an emeritus professor at the UCL Mullard Space Science Laboratory, said: “The polarization at low energies tells us that the magnetic field is probably strong enough to turn the atmosphere around the star into a solid or liquid substance, 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.
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 hard crust is a plausible explanation. Professor Jeremy Hale from the University of British Columbia (UBC) added: “It is also worth noting that including the effects of quantum electrodynamics, as we did in our theoretical modeling, gives results consistent with the IXPE observation. However, we are also exploring alternative models to explain the IXPE data, for which adequate numerical simulations are still lacking.” (ANI)
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