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Research: Magnetized dead star has solid surface

ANI | Updated: Nov 04, 2022 23:59 IST


Padova [Italy], November 4 (ANI): The research, headed by scientists at the University of Padova and published in the journal Science, makes use of information from the Imaging X-ray Polarimetry Explorer (IXPE), a NASA satellite that was launched in December of last year.
By detecting polarisation, or the direction of the light waves' wobble, the satellite--a joint project of NASA and the Italian Space Agency--offers a fresh perspective on X-ray photons in space.
The group examined the magnetar 4U 0142+61 observation from IXPE, which is situated around 13,000 light years from Earth in the Cassiopeia constellation. It had never before been possible to see polarised X-ray photons coming from a magnetar.
The extremely dense remaining cores of massive stars that have burst as supernovae at the conclusion of their lives are known as magnetars. They have a huge magnetic field, the strongest in the cosmos, unlike other neutron stars.
They produce brilliant X-rays and exhibit irregular activity, emitting bursts and flares that can discharge energy millions of times higher than what our Sun releases in a year in just one second. They are thought to be propelled by their extraordinarily strong magnetic fields, which are 100-1,000 times stronger than those of typical neutron stars.
When compared to what would be predicted if the X-rays went through an atmosphere, the research team discovered a far smaller fraction of polarised light. (Polarized light is light in which the electric fields vibrate only in one direction, or in which the wiggle is completely in one direction.) An atmosphere serves as a filter, limiting the light's polarisation states to one.)
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 is in agreement with what theoretical models predict and confirms that magnetars are indeed endowed with ultra-strong magnetic fields."
According to quantum theory, light will be polarised in two directions--parallel and perpendicular to the magnetic field--as it travels through a strongly magnetised environment. 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 condition of materials in the region of the neutron star.
The observed 90-degree polarisation swing is consistent with the expectation that photons (particles of light) polarised perpendicular to the magnetic field will predominate at high energy.
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 magnetic field is assumed to hold an ion lattice, which makes up the star's solid crust, together. The atoms would be stretched in the magnetic field's direction rather than spherical.
The existence of atmospheres around magnetars and other neutron stars is still up for dispute. The latest study is the first detection of a neutron star, though, for which a reliable explanation including a solid crust can be offered.
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 modelling, 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." (ANI)

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