First detection of X-ray polarisation from a cataclysmic variable
For the very first time, scientists have measured the polarisation of X-rays emitted by a white dwarf cannibalising its stellar companion. This observation is an important step forward in the study of the dynamics of these celestial bodies and confirms how matter is transferred in a binary.
A white dwarf is a dead sun that has consumed all its nuclear fuel. The star has expelled its outer layers, leaving only a very dense corpse – the size of Earth but with a mass close to that of our Sun. Some white dwarfs form a binary system, a pair of objects bound to each other. Located 200 light-years away, EX Hydrae is one such binary system. It consists of a white dwarf and a second star, still very much alive, and has been the target of an international research group - led by the MIT - including Yaël Nazé, astrophysicist and FNRS senior research fellow at GAPHE at ULiège (STAR Institute).
"In the system we chose, the extremely dense white dwarf attracts matter from its neighbour," explains the researcher. "It's true stellar cannibalism! This type of very close pair is called a cataclysmic variable (CV)." The matter torn from the companion star first forms a disc around the white dwarf but there is another important factor: the white dwarf has a powerful magnetic field. "At a certain point, the approaching matter can no longer move around freely, as the magnetic field redirects it towards the magnetic poles of the white dwarf."
As it approaches the surface, the gas falls onto the dead star at speeds of millions of kilometres per hour and forms a kind of heated column - about 3,000 kilometres high, roughly half the radius of the white dwarf. There, the matter is suddenly slowed down and heated to tens of millions of degrees. It then emits powerful X-rays. "We had suspected this theoretical scenario for a long time. But until now, there was no direct evidence of the shape and orientation of this region."
Seeing the invisible thanks to polarisation
This is where polarisation comes in. The word is a bit technical, but the principle is quite familiar. Natural light actually vibrates in all directions, but reflections only vibrate in a particular direction. This is how polarised sunglasses block glare. The X-rays emitted by EX Hydrae behave in the same way. When these X-rays are produced in the gas column, they reflect off the surface of the white dwarf and their direction of oscillation is no longer random. "Measuring this polarisation allows us to know exactly where the X-rays come from and what structure they have bounced off."
This had never been done before, but thanks to the American space telescope IXPE (Imaging X-ray Polarimetry Explorer), the team obtained the first measurement of X-ray polarisation for a cataclysmic variable/this type of cannibalistic system. The result: not only is the signal polarised, but its direction corresponds to that of the poles. The team was then able to estimate the height of the shock where the incoming matter crashes into the column: about half the radius of the white dwarf. This refines the models, makes the simulations more realistic and helps to better understand how these systems evolve until, perhaps, the final explosion.
With this world first in X-ray polarimetry of an accreting white dwarf, the community finally has a true "map" of the most violent region of a cataclysmic variable system. "We have moved from a theoretical scenario to solid observational evidence," concludes Yaël Nazé. This first opens the way to the study of other white dwarfs and, ultimately, to a better understanding of the evolution of stellar pairs.
Scientific reference
Sean J. Gunderson, Swati Ravi, Herman L. Marshall, Dustin K. Swarm, Richard Ignace, Yaël Nazé, David P. Huenemoerder, Pragati Pradhan, X-ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae, The Astrophysical Journal, 2025 993 248. DOI 10.3847/1538-4357/ae11b5
- MIT : Sean Gunderson, Swati Ravi, Herman Marshall & David Huenemoerder
- University of Iowa : Dustin Swarm
- East Tennessee State Universit : Richard Ignac
- Université de Liège : Yaël Nazé
- Embry Riddle Aeronautical University : Pragati Pradhan
