MIT astronomers utilized a space-based X-ray telescope to delineate critical features within a star system’s innermost region, an exceptionally energetic domain previously inaccessible to most observational instruments. In an open-access study published in the Astrophysical Journal, the research team reported their use of NASA’s Imaging X-ray Polarimetry Explorer (IXPE) to observe the intermediate polar designated EX Hydrae.
A smaller white dwarf star (left) pulls material from a larger star into a swirling accretion disk. The pair is called an “intermediate polar,” and MIT astronomers used powerful telescopes to measure the system’s X-ray polarization for the first time, revealing key features at the center of its hottest, most extreme regions. Image Credit: Jose-Luis Olivares, MIT
Located approximately 200 light-years from Earth, a white dwarf, representing the core of a dead star, orbits a larger companion star. This white dwarf possesses a potent magnetic field, drawing material from its companion into a swirling, accreting disk. This binary system is classified as an "intermediate polar," characterized by its emission of complex, intense radiation, including X-rays, as gas from the larger star accretes onto the white dwarf.
The investigation revealed a remarkably high degree of X-ray polarization, which quantifies the orientation of an X-ray wave’s electric field, alongside an unanticipated polarization direction in the X-rays originating from EX Hydrae. These precise measurements enabled researchers to trace the X-rays directly to their source within the system's innermost region, in close proximity to the white dwarf's surface.
The team determined that the system’s X-rays emanated from a column of incandescent material being drawn from the companion star by the white dwarf. This accretion column is estimated to be approximately 2,000 miles high, roughly half the white dwarf’s radius and significantly taller than theoretical predictions for such systems.
The study also confirmed that X-rays reflect off the white dwarf’s surface before dispersing into space, an effect previously hypothesized by physicists but unconfirmed until this research.
The study conclusively demonstrates the efficacy of X-ray polarimetry as a robust method for investigating extreme stellar environments, particularly the most energetic regions of accreting white dwarfs.
We showed that X-ray polarimetry can be used to make detailed measurements of the white dwarf's accretion geometry. It opens the window into the possibility of making similar measurements of other types of accreting white dwarfs that also have never had predicted X-ray polarization signals.
Sean Gunderson, Study Lead Author and Postdoc, Kavli Institute for Astrophysics and Space Research, MIT
Gunderson’s MIT Kavli co-authors include graduate student Swati Ravi and research scientists Herman Marshall and David Huenemoerder, along with Dustin Swarm of the University of Iowa, Richard Ignace of East Tennessee State University, Yael Nazé of the University of Liège, and Pragati Pradhan of Embry Riddle Aeronautical University.
A High-Energy Fountain
All forms of light, including X-rays, are influenced by electric and magnetic fields. Light travels in waves that wiggle, or oscillate, at right angles to the direction in which the light is traveling. External electric and magnetic fields can pull these oscillations in random directions. When light interacts and bounces off a surface, it can become polarized, meaning that its vibrations tighten up in one direction.
Polarized light can help scientists identify the origin of the light and reveal details about the source’s shape and structure.
Launched in 2021, NASA's Imaging X-ray Polarimetry Explorer (IXPE) is a groundbreaking space observatory dedicated to studying polarized X-rays from some of the universe's most extreme environments. Orbiting the Earth, IXPE captures and analyzes these X-rays, primarily focusing on objects like supernovae, black holes, and neutron stars.
This groundbreaking MIT study marks the first application of the IXPE to measure polarized X-rays from an intermediate polar. This type of system, smaller than black holes and supernovas, is still a potent source of X-rays.
We started talking about how much polarization would be useful to get an idea of what’s happening in these types of systems, which most telescopes see as just a dot in their field of view.
Herman Marshall, Research Scientist, MIT
The designation of an intermediate polar system is determined by the magnetic field strength of its central white dwarf. In scenarios where this field is robust, material from the companion star is directly channeled towards the white dwarf’s magnetic poles. Conversely, when the field is significantly attenuated, stellar material instead forms an accretion disk orbiting the dwarf, eventually depositing matter onto its surface.
For an intermediate polar, theoretical models predict a complex, hybrid accretion pattern. This involves the formation of an accretion disk that is subsequently drawn towards the white dwarf’s poles.
The magnetic field is expected to elevate the incoming material disk substantially, akin to a high-energy fountain, before the stellar debris descends towards the white dwarf’s magnetic poles. This descent occurs at velocities reaching millions of miles per hour, a phenomenon termed an “accretion curtain.”
It is further hypothesized that this infalling material will collide with previously lifted, still-descending matter, creating a localized compression zone. This accumulation of matter is projected to form a column of colliding gas, attaining temperatures in the tens of millions of degrees Fahrenheit, and consequently emitting high-energy X-rays.
An Innermost Picture
The team's objective was to test the hypothesized model of intermediate polars through the measurement of polarized X-rays from EX Hydrae. To achieve this, IXPE conducted approximately 600,000 seconds (seven days) of X-ray observations of the system in January 2025.
“With every X-ray that comes in from the source, you can measure the polarization direction. You collect a lot of these, and they’re all at different angles and directions, which you can average to get a preferred degree and direction of the polarization,” explained Marshall.
The system's X-ray emission exhibits an 8 % polarization degree, a value significantly exceeding theoretical model predictions. This characteristic confirms that the X-rays originate from the system's column, which has been precisely measured at approximately 2,000 miles in height.
“If you were able to stand somewhat close to the white dwarf’s pole, you would see a column of gas stretching 2,000 miles into the sky, and then fanning outward,” said Gunderson.
The team also measured the direction of EX Hydrae’s X-ray polarization, which they determined to be perpendicular to the white dwarf’s column of incoming gas. This was a sign that the X-rays emitted by the column were then bouncing off the white dwarf’s surface before traveling into space, and eventually into IXPE’s telescopes.
“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. When we look through other telescopes, we don’t see any of this detail,” said Ravi.
The research program will utilize X-ray polarization to study diverse accreting white dwarf systems. This approach is anticipated to provide critical data for understanding accretion mechanisms and their broader relevance to cosmic phenomena.
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. So understanding these white dwarf systems helps scientists understand the sources of those supernovae, and tells you about the ecology of the galaxy.
Herman Marshall, Research Scientist, MIT
The study was supported by NASA.
Journal Reference:
Pradhan, P., et al. (2025) X-Ray Polarimetry of Accreting White Dwarfs: A Case Study of EX Hydrae. Astrophysical Journal. DOI:10.3847/1538-4357/ae11b5. https://iopscience.iop.org/article/10.3847/1538-4357/ae11b5