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In the cosmic graveyard of dead stars, a magnetic "zombie" is putting on a spectacular show, forcing astronomers to rethink their models. NASA's Imaging X-ray Polarization Explorer (IXPE) spacecraft recently turned its gaze to a peculiar stellar system, EX Hydrae, and what it saw was far more twisted and brilliant than expected.
For nearly a week in 2024, IXPE dedicated its time to studying this system, located a relatively close 200 light-years from Earth. EX Hydrae is what's known as an intermediate polar—a type of binary system where a white dwarf, the ultra-dense corpse of a sun-like star, orbits a living companion star.
The drama unfolds as the white dwarf’s intermediate-strength magnetic field acts as a cosmic thief, pulling material away from its normal companion star. In systems with immensely strong magnetic fields, this stolen gas gets funneled directly to the white dwarf's poles. In weaker systems, the gas forms a swirling disk around it. But EX Hydrae, true to its "intermediate" name, does both: some material forms an accretion disk, while other streams are yanked directly toward the magnetic poles in a violent, energetic process.
IXPE's unique mission is to measure the polarization of X-rays from space, a property that reveals details about the geometry and magnetic environment of their source. When it looked at EX Hydrae, it delivered a one-two punch of surprises.
First, the intensity of the polarization was staggering. The data revealed an 8% polarization degree in the X-ray light. "This was much higher than some models had predicted," researchers noted, indicating the environment is far more ordered and structured than previously thought.
Second, IXPE pinpointed the origin and path of these powerful X-rays. They were found to erupt from a towering column of super-heated gas, channeled from the inner edge of the accretion disk onto a small, blazing-hot spot on the white dwarf's surface. The team estimated this column to be about 2,000 miles high—a staggering structure that, much like the polarization measurement, was far larger than theories had suggested.
But the final twist came from the direction of the polarized light. The team discovered the X-ray polarization was oriented perpendicular to the column of incoming gas. This crucial clue solved a puzzle: the X-rays aren't streaming straight out of the column into space. Instead, they fire down onto the surface of the white dwarf itself, reflect off its dense crust, and then scatter into the cosmos where IXPE can detect them.
You can dive into the full details of this discovery in the team’s published study in The Astrophysical Journal here.
"This is a fascinating system and a testament to the new kind of information IXPE is giving us," said a researcher involved in the study. "We’re not just seeing how bright these objects are, but getting a deeper understanding of their fundamental shape and magnetic fields." For more on IXPE's groundbreaking work, NASA provides an overview of the mission's achievements here.
The team now plans to use IXPE's eye for polarization to investigate more white dwarf systems. By doing so, they hope to crack the code on the complex physics of accretion and magnetism—knowledge that could shed light on much larger-scale cosmic events, including the violent interactions of neutron stars and the growth of supermassive black holes. The humble white dwarf, it seems, still has profound lessons to teach us.