A group of scientists recently published a study in the journal Science suggesting that a dead magnetized star likely has a solid surface.
The research led by UCL researchers found that densely magnetized dead stars, also called magnetars, emit X-ray light that indicates they are solid without atmospheres. NASA’s Imaging X-ray Polarimetry Explorer (IXPE) satellite, launched last December, provided data for the study published in the journal Science. It measures the polarisation of X-ray light in space and the direction in which it wiggles. In this study, the team examined IXPE’s observation of magnetar 4U 0142+61. It is located in the Cassiopeia constellation, 13,000 light years from Earth. The study was the first to observe polarized X-ray light from a magnetar.
At the end of their life, massive stars explode as supernovae, leaving behind neutron stars. They have the most powerful magnetic fields in the universe, unlike other neutron stars. These stars are known for erratic periods of activity, accompanied by bright X-rays and flashes of energy that are millions of times more powerful than those produced by our Sun each year in just one second. Unlike standard neutron stars, they are believed to be powered by ultra-powerful magnetic fields. According to the research team, X-rays passing through an atmosphere would produce a much higher proportion of polarised light. A polarized light is light with all the wiggles moving in the same direction – that is, the electric fields are only vibrating in one direction. The atmosphere acts as a filter, limiting the amount of polarization.
The team also discovered that light particles with higher energies have their angle of polarisation flipped by 90 degrees. It is interesting to note that theoretical models predicted something similar if a star had a solid crust surrounded by an external magnetosphere. Prof. Silvia Zane, who is the co-lead author and a member of the IXPE science team, explained that this was entirely unexpected. In fact, scientists were convinced that their observations would definitely reveal the existence of an atmosphere. But this was not the case. Similarly to how water would turn to ice after reaching a tipping point, the star’s gas has solidified. The star’s magnetic field is extremely strong, causing this.
“But, as with water, the temperature is also an imperative component. Hotter gas will need more magnetic field strength to solidify.” Observing hot neutron stars with similar magnetic fields would help scientists better understand this process. Researchers wanted to understand how the star’s surface properties are affected by the interaction between temperature and magnetic field. Professor Roberto Taverna, the lead author of the study from the University of Padova, said the most fascinating feature was the 90-degree swing in polarisation direction with energy.
In agreement with theory
“This agrees with what theoretical models predict and confirms that magnetars are indeed endowed with ultra-strong magnetic fields.” There are two directions in which polarised light is produced in a strongly magnetized environment, parallel and perpendicular to the magnetic field. These two directions are the directions in which light propagates according to quantum theory. When polarization is observed, it reflects the magnetic field structure and the physical state of matter surrounding a neutron star. This provides information that was not otherwise possible. It is expected that photons (particles of light) polarized perpendicularly to the magnetic field dominate at high energies. This leads to the 90-degree polarisation swing observed.
As honorary professor at the Mullard Space Science Laboratory at UCL, professor Roberto Turolla explained, polarization at low energies indicates that the magnetic field of a star is probably strong enough to cause its atmosphere to condense, a phenomenon called magnetic condensation. Magnetic fields hold together the ions forming the star’s solid crust. Magnetic fields would cause atoms to elongate in the direction of the magnetic field instead of being spherical. It is still debated whether magnetars and other neutron stars have atmospheres. Despite this, this is the first observation of a neutron star providing a reliable explanation for its solid crust.