Astronomers peek into innermost regions of white dwarf binary system
White dwarfs, the cores of dead stars, vampirically draw material from the outer layers of binary companions. For the first time, astronomers have used powerful telescopes to peer into the hottest, most extreme regions of such systems.
A team of astronomers from MIT have used NASA's X-ray Polarimetry Explorer (IXPE) to observe an eclipsing intermediate polar cataclysmic variable star designated as EX Hydrae, at a distance of 200 lightyears from the Earth. The system consists of a highly magnetised core of a dead star, known as a white dwarf that is orbiting a larger binary companion, drawing in material from its outer atmosphere in a swirling accretion disk. The star system gives off a complex pattern of intense radiation, as the gas from the larger star falls into the smaller star, which is why it is known as an 'intermediate polar system'. This is the first time that astronomers have peered into the innermost, chaotic regions of the system.
The astronomers discovered a high degree of X-ray polarisation, which is the electrical field of the X-rays, as well as unexpected polarisation directions. The X-rays were traced back to close to the surface of the white dwarf, which is extremely flat because of the immense gravity. The tallest 'mountains' on this compact body is just a few centimetres high! The astronomers were able to determine that the X-rays from the system were emitted from a column of superheated material that the white dwarf is pulling from this binary companion. This column 3.21 km high, about half the radius of the white dwarf itself, and much taller than theoretical predictions for such systems. The research also revealed that the surface of the white dwarf reflects the X-rays.
Mapping the accretion of white dwarfs
Scientists had suspected that the surfaces of white dwarfs reflect X-rays before they are scattered in space, but have observed the process in action for the first time now. A paper describing the research has been published in The Astrophysical Journal. Lead author of the paper, Sean Gunderson says, "We showed that X-ray polarimetry can be used to make detailed measurements of the white dwarf's accretion geometry. It opens the window into the possibility of making similar measurements of other types of accreting white dwarfs that also have never had predicted X-ray polarization signals." The OXPE captured about seven days of X-ray measurements from the system in January 2025. The astronomers plan to study other white dwarf systems next.