A weird dead star may have a solid-like surface made of iron crystals

X-ray observations show that a distant magnetar may have a surface made of a strange material somewhere between a solid and a fluid instead of a gaseous one like most stars

By Leah Crane

A strange star about 13,000 light years away may have a solid surface, not a gaseous one like most stars. The star, called 4U 0142+61 but nicknamed simply 4U, is a magnetar – the dense, highly magnetised corpse of a giant star that exploded in a supernova – and researchers have never seen one like this before.

Roberto Turolla at the University of Padova in Italy and his colleagues used a NASA satellite called the Imaging X-ray Polarimetry Explorer (IXPE) to examine X-rays from the magnetar. Specifically, they looked at the polarisation of the light – the plane in which the light waves oscillate as they propagate through space. This depends heavily on the medium through which the waves are propagating, as well as electromagnetic fields, so they were able to use it to characterise 4U’s surface and the area directly around it.

They found two strange properties that suggested the magnetar might have a solid surface. The first was that the polarisation of the light was very weak, whereas a gas would have acted as a filter and allowed mostly polarised light through. The second was a sudden 90 degree switch in the polarisation angle, exactly what simulations predict for X-rays travelling out through a solid surface directly into a space empty of everything but electromagnetic fields.

Because of the extreme gravity at the surface of the magnetar, this strange shell is most likely a type of material called a Bingham plastic, which is somewhere between a solid and a fluid – mayonnaise and toothpaste are two common examples. But instead of being made of anything we would commonly associate with fluids, it is probably made of strange crystals of iron and other heavy elements, their atoms stretched into cigar shapes by the powerful magnetic fields, says Turolla.

While this star does seem to have a surface, walking on it is out of the question because the gravity at your feet would be so much stronger than the gravity at your head. “There is this huge difference in the gravity, which would definitely kill you because it would spaghettify you. Then there is all the radiation and the extreme heat,” says Turolla. “Even getting close would be very dangerous.”

It is not yet clear whether we should expect solid surfaces like this to be common in magnetars. The researchers chose 4U because this type of observation requires a lot of light and it is one of the brightest magnetars in the sky, but the observations still took nearly a month. To gather enough data to figure out whether other magnetars’ surfaces are solid or gaseous, we may have to wait a while.

Journal reference: Science, DOI: 10.1126/science.add0080

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