MIT Scientists Capture the First Detailed Look Inside an Accreting White Dwarf System Using X-Ray Polarization

A team of researchers has achieved something astronomers have been trying to do for decades: directly probe the innermost, most energetic region of an accreting white dwarf system. Using NASA’s Imaging X-ray Polarimetry Explorer (IXPE), scientists studied the system EX Hydrae, located about 200 light-years from Earth, and uncovered a level of detail never before observed in such systems.

EX Hydrae is what astronomers call an intermediate polar, a type of binary star system in which a white dwarf — essentially the hot core left behind after a star dies — pulls material from a larger companion star. This infalling material doesn’t simply fall straight onto the dwarf. Instead, it forms a swirling accretion disk, shaped and distorted by the white dwarf’s moderately strong magnetic field. As this material spirals inward, it releases significant energy, including intense X-rays. Until now, the internal structure of this environment was mostly theoretical due to its extremely small size and high-energy nature.

The new IXPE observations allowed researchers to measure the polarization of the emitted X-rays — the direction in which the X-ray light waves vibrate. Polarization carries geometric information, meaning it reveals the shape, orientation, and behavior of the region where the X-rays were produced and scattered. The study recorded an unexpectedly high 8% polarization degree, which is notable because older theoretical models predicted only minimal polarization in such systems.

From this measurement, scientists identified the X-rays’ origin: a column of superheated gas towering about 2,000 miles above the white dwarf’s surface. This accretion column, created by gas funneled along magnetic field lines, is roughly half the radius of the white dwarf — far taller than expected. Previous models suggested much shorter columns, but the new results show the structure is not only larger but dynamically more complex.

What makes the finding even more significant is the direction of the detected polarization. The polarization angle was found to be perpendicular to the direction of the incoming accretion flow. This pattern strongly indicates that after the gas emits X-rays, those X-rays bounce off the surface of the white dwarf before escaping into space. Astronomers long suspected such “reflection effects,” but this is the first direct observational confirmation.

To obtain this data, IXPE spent about 600,000 seconds (roughly seven days) observing EX Hydrae in January 2025. With each incoming X-ray photon, IXPE measured the orientation of its electric field. When combined, the readings revealed the system’s preferred polarization direction and the strength of the polarization signature, giving researchers a clear geometric map of this previously invisible region.

The system’s behavior matches what physicists expect from an intermediate polar. In systems with strong magnetic fields, matter is funneled directly onto the white dwarf’s magnetic poles. In systems with weak magnetic fields, matter spreads out across the white dwarf via a stable accretion disk. Intermediate polars form an in-between category, where the disk exists but is disrupted near the white dwarf and lifted into magnetic “funnels.” These funnels create what astronomers call an accretion curtain, where gas plummets toward the white dwarf’s poles at millions of miles per hour. The falling gas slams into slower-moving material near the surface, producing a shock that heats the region to tens of millions of degrees Fahrenheit. This is the source of the observed X-rays.

Understanding this region is essential because accreting white dwarfs play a significant role in stellar evolution and cosmic events. If a white dwarf accumulates too much material, it can collapse and explode into a Type Ia supernova, one of the most important tools astronomers use to measure cosmic distances and study the expansion of the universe. Knowing how white dwarfs accumulate material — and how efficiently they convert gravitational energy into X-rays — helps refine models of how these supernovae form.

The EX Hydrae study also demonstrates that X-ray polarimetry is a powerful tool for understanding accreting systems. Until now, IXPE’s targets were mostly black holes, neutron stars, and supernova remnants — all much larger or more intense than intermediate polars. This research shows that polarimetry can be applied to smaller but still extreme objects, opening the door to studying many other white dwarf binaries.

The research team included scientists from MIT’s Kavli Institute for Astrophysics and Space Research, the University of Iowa, East Tennessee State University, the University of Liège, and Embry Riddle Aeronautical University. Their coordinated effort not only confirmed long-standing predictions about accretion structures but also revealed new, unexpected behavior that will force a rethinking of existing models.

Below are additional sections to expand your readers’ understanding of the topic.

What Exactly Is a White Dwarf?

A white dwarf is the leftover core of a star that once resembled our Sun. After exhausting its nuclear fuel, the star sheds its outer layers and leaves behind a dense, Earth-sized core composed mostly of carbon and oxygen. White dwarfs are incredibly dense — a teaspoon of their material would weigh several tons on Earth — and they no longer undergo nuclear fusion. Instead, they slowly cool over billions of years.

White dwarfs often appear in binary systems, and if they have a close companion, they may steal material from it. This process of mass transfer can dramatically alter their structure and evolution.

Understanding Intermediate Polars

Intermediate polars (IPs) form a subset of cataclysmic variables, binary systems in which a white dwarf accretes matter from a companion star. The defining factor is the moderate magnetic field strength of the white dwarf. It is strong enough to disrupt part of the accretion disk but not strong enough to eliminate it entirely.

In an IP like EX Hydrae, this creates a hybrid accretion structure:

a partially intact accretion disk
an inner region disrupted by magnetic fields
multiple accretion curtains funneling gas toward the poles
a tall, shock-producing accretion column

These complex interactions generate optical, ultraviolet, and high-energy X-ray emissions, making IPs valuable for testing theories of magnetically controlled accretion.

Why X-Ray Polarization Is Important

Traditional X-ray observations provide data about intensity and energy spectrum, but they cannot reveal geometry. X-ray polarization, however, is highly sensitive to:

scattering angles
magnetic field structure
emission geometry
surface reflections

This is why IXPE’s detection of high polarization in EX Hydrae is so important — it provides direct clues about the shape and orientation of the accretion column and the structure of the white dwarf’s surface.

What This Means for Future Research

The successful use of X-ray polarimetry on EX Hydrae sets the stage for:

studying other intermediate polars
investigating magnetic white dwarfs with stronger fields
exploring how accretion geometry evolves over time
refining models of white dwarf mass growth
improving simulations of Type Ia supernova precursors

It also highlights how much we still have to learn about accretion physics in compact systems. Even relatively small objects like white dwarfs are capable of producing incredibly energetic, complex, and dynamic environments.