White Dwarf’s Hidden Furnace Finally Comes Into View

For the first time, astronomers have watched the innermost furnace of a dying star’s feeding frenzy imprint itself on escaping X rays.

Roughly 200 light years from Earth, a dense white dwarf in the system EX Hydrae is locked in orbit with a larger companion star. The dead core is not quiet. It siphons gas from its neighbor, building a bright, swirling accretion disk and flinging high energy light across space. Using NASA’s Imaging X ray Polarimetry Explorer (IXPE), an international team led by MIT scientists has now measured how those X rays are polarized, and used that pattern to map the system’s most extreme, previously hidden regions.

The results, published in The Astrophysical Journal, show that the X rays from EX Hydrae are surprisingly strongly polarized, about 8 percent in the 2 to 3 kiloelectronvolt energy band. That number sounds small, but for astrophysical X rays it is striking. From that subtle tilt in the light’s electric field, the team inferred that most of the X rays come from a narrow column of superheated gas plunging onto the white dwarf and then bouncing off the star’s surface before streaming into space.

Peering into an intermediate polar

EX Hydrae belongs to a class of systems known as intermediate polars, where a white dwarf’s magnetic field is strong enough to disrupt the inner part of the accretion disk, but not so strong that the disk disappears entirely. Gas from the companion star first orbits in the disk, then is grabbed by magnetic field lines and lifted into a kind of high energy fountain that rains back down near the white dwarf’s magnetic poles. Astronomers have long suspected that this fall forms a standing shock, a column of gas heated to tens of millions of degrees that should shine in hard X rays.

Until now, that inner geometry has been nearly impossible to see. In typical X ray images, EX Hydrae is just a bright point. Spectra reveal that hot gas is present, but not exactly where or how it is arranged. Polarization adds two new pieces of information: the preferred direction of the light’s oscillation and how strong that preference is. Those two numbers, measured across many photons, carry a fingerprint of the configuration of plasma and magnetic fields at the very center of the system.

IXPE observed EX Hydrae for about 600,000 seconds in January 2025, gathering enough photons to pick out a clear polarization signal in the softest part of its band. The team then compared the measured degree and angle of polarization with models of how X rays should scatter off the white dwarf’s surface. The picture that emerged is dramatic. The accretion column towering above the magnetic pole appears to be roughly 2,000 miles high, about half the radius of the white dwarf itself and significantly taller than many theorists had expected. The orientation of the polarization also shows that a substantial fraction of the X rays are reflected off the star’s surface very close to the base of that column.

“The thing that’s helpful about X ray polarization is that it’s giving you a picture of the innermost, most energetic portion of this entire system,” Ravi says. “When we look through other telescopes, we don’t see any of this detail.”

“There comes a point where so much material is falling onto the white dwarf from a companion star that the white dwarf can’t hold it anymore, the whole thing collapses and produces a type of supernova that’s observable throughout the universe, which can be used to figure out the size of the universe,” Marshall offers. “So understanding these white dwarf systems helps scientists understand the sources of those supernovae, and tells you about the ecology of the galaxy.”

A new tool for extreme stellar weather

Behind the scenes, the analysis leans into sophisticated statistics. The team combined a model independent measurement of polarization with Bayesian methods that test whether the data truly require a polarized signal at all. The answer, in the 2 to 3 keV band, is yes. At higher energies, IXPE did not detect significant polarization, which the authors attribute to lower signal to noise and to the fact that the hottest, highest energy photons are produced only near the very top of the column, where fewer X rays are emitted overall.

By adapting a so called lamppost geometry often used to describe black holes, the researchers showed that the measured polarization can be translated directly into the height of the accretion shock above the white dwarf’s surface. Their best estimate, about half a stellar radius, agrees with previous constraints based on X ray spectra alone, but now comes from an entirely independent line of evidence that does not depend on uncertain details of the inner accretion disk.

The work also sketches a path forward. EX Hydrae is bright and nearby, which makes it an ideal first case, but it is unlikely to be unique. Other accreting white dwarfs, including systems that do not show strong optical polarization, may still reveal their inner geometry when viewed through X ray polarimetry. Longer IXPE observations and future soft X ray polarimeters could slice the data by spin phase, watching how the polarization changes as the white dwarf rotates and the accretion curtain swings through view.

For now, the EX Hydrae study shows that even a compact stellar remnant can host a surprisingly tall and intricate atmosphere of inflowing gas, shaped by magnetic fields and recorded in the delicate orientation of X rays. By reading that pattern, astronomers are beginning to turn a point of light into a three dimensional physical object, one accreting white dwarf at a time.

Journal: The Astrophysical Journal, “X Ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae” (DOI: 10.3847/1538-4357/ae11b5).