For the first time, astronomers have used NASA’s Imaging X-ray Polarization Explorer (IXPE) to measure and map the environment around a white dwarf star. The target was an energetic binary system known as an intermediate polar, EX Hydrae, located roughly 200 light-years away in the constellation Hydra.
A white dwarf is the incredibly dense core left behind when a Sun-like star exhausts its nuclear fuel. In the EX Hydrae system, this Earth-sized stellar remnant, holding a Sun’s worth of mass, is locked in a gravitational dance with a normal companion star. Gas from this companion star spirals toward the white dwarf, forming what is known as an accretion disk.
The key to understanding this chaotic process is the white dwarf’s magnetic field. In EX Hydrae, the magnetic field is strong enough to disrupt the inner part of the disk and funnel some infalling gas directly toward its magnetic poles, but not strong enough to completely control the flow. This “intermediate” behavior defines the system’s classification. As this gas smashes onto the white dwarf’s surface at nearly a quarter of the speed of light, it is heated to tens of millions of degrees, producing powerful X-rays.
In 2024, IXPE observed EX Hydrae for nearly a week. Its unique ability to measure the polarization of X-rays—the direction their electric fields vibrate—allowed the team to deduce the geometry of this violent environment with unprecedented precision, without having to directly image features too small to see.
“IXPE’s polarimetry allowed us to measure the height of the accreting column of gas to be almost 2,000 miles high, with fewer assumptions than past methods,” said lead author Sean Gunderson of MIT. “The data suggest the X-rays we observed likely scattered off the white dwarf’s surface itself, revealing details on a scale we could never hope to image directly.”
The results, published in The Astrophysical Journal by a team led by MIT with co-authors from several universities, provide a new template for understanding how matter behaves in the powerful magnetic fields of white dwarfs and similar compact objects like neutron stars. This landmark observation demonstrates how X-ray polarimetry can unlock the hidden structure of some of the universe’s most dynamic and extreme systems.
