SAEDNEWS: Using NASA's Imaging X-ray Polarimetry Explorer (IXPE) spacecraft, astronomers have obtained their first view of the inner region around a dead white dwarf star that is vampirically feeding on a stellar companion.
A team from the Massachusetts Institute of Technology (MIT) has conducted a detailed study of the previously inaccessible, highly energetic region surrounding a white dwarf in the EX Hydrae system, located roughly 200 light-years from Earth.
EX Hydrae belongs to a class of binary star systems called “intermediate polars,” known for emitting complex patterns of radiation, including X-rays. The system consists of a white dwarf—the dense remnant of a sun-like star—and its companion star, which orbits the white dwarf every 98 minutes, making EX Hydrae one of the closest intermediate polar binaries ever discovered.
The researchers not only detected a high degree of polarization in the X-rays—showing alignment in the direction of the waves that make up electromagnetic radiation—but also traced this energetic radiation to a 2,000-mile-tall (3,200 kilometers) column of blistering-hot stellar material being pulled from the companion star and crashing onto the white dwarf. That column is about half the radius of the white dwarf itself, far larger than scientists had previously estimated for such structures. They also observed X-rays reflecting off the white dwarf’s surface before scattering, confirming a long-predicted phenomenon.
Intermediate polars earned their name from the varying strengths of white dwarfs’ magnetic fields. Strong magnetic fields funnel material from companion stars to the white dwarfs’ poles, while weaker fields form swirling accretion disks. In systems like EX Hydrae, with intermediate-strength fields, accretion disks are dragged toward the poles, creating “accretion curtains”—fountains of stellar matter raining down at millions of miles per hour. Colliding streams of falling material can heat gas to millions of degrees, producing X-rays.
In January 2025, MIT’s team tested this theory with roughly seven Earth-days of observations using NASA’s Imaging X-ray Polarimetry Explorer (IXPE). Their findings highlight the power of X-ray polarimetry—a technique that measures the polarization of X-rays—in probing extreme stellar environments.
“X-ray polarimetry allows us to map the accretion geometry of the white dwarf in unprecedented detail,” said team leader Sean Gunderson of MIT’s Kavli Institute for Astrophysics and Space Research. “It opens the door to similar measurements in other accreting white dwarfs, which previously lacked predicted X-ray polarization signals.”
Polarized Insights
Light waves oscillate perpendicular to their direction of travel, and magnetic or electric fields can influence this angle. Light bouncing off surfaces can also become polarized, meaning its oscillation aligns in a preferred direction. By studying polarized light, researchers can learn more about the source it scattered from.
Launched in 2021, IXPE is NASA’s first mission designed to detect polarized X-rays and has studied some of the universe’s most extreme objects, including neutron stars, black holes, and supernovae. EX Hydrae represents the first intermediate polar system studied by IXPE—a smaller but still powerful X-ray emitter.
MIT scientists found an 8% polarization degree in EX Hydrae’s X-rays, much higher than theoretical predictions. This confirmed that the radiation originates from a towering 2,000-mile column of colliding gas. “If you stood near the white dwarf’s pole, you’d see this massive column stretching skyward, then fanning outward,” Gunderson said.
Measuring the X-rays’ polarization confirmed that the radiation bounces off the white dwarf’s surface before traveling into space. “X-ray polarization gives a window into the most energetic, innermost parts of this system,” added MIT scientist Swati Ravi. “Other telescopes simply can’t see this level of detail.”
Looking ahead, the team plans to expand their study to other “vampire” white dwarf systems, potentially shedding light on their ultimate fate—Type Ia supernovae, which result from the overfeeding of white dwarfs and can help measure the size of the universe.
“There comes a point when a white dwarf can’t hold any more material from its companion. It collapses, producing a supernova visible across the cosmos,” said team member Herman Marshall. “Studying these systems helps us understand the sources of these cosmic explosions and the ecology of our galaxy.”
