NASA’s IXPE mission has, for the first time, measured a white dwarf star, using its unique X-ray polarimetry to map the high-energy geometry of an “intermediate polar” system known as EX Hydrae. Located about 200 light-years away, this binary system features a white dwarf—an Earth-sized stellar remnant as massive as the Sun—locked in a dynamic dance with a companion star.
Gas from the companion star spirals toward the white dwarf, forming an accretion disk. However, the white dwarf’s magnetic field, while not overwhelmingly strong, is powerful enough to disrupt this flow, pulling some material directly toward its magnetic poles. This creates a chaotic, superheated environment where infalling matter slams into the star’s surface at millions of degrees, generating intense X-rays.
For nearly a week in 2024, IXPE trained its instruments on EX Hydrae. Its special capability to measure the polarization of X-rays—essentially the orientation of their light waves—allowed astronomers to deduce physical structures far too small to be seen directly through imaging.
The key finding, led by researchers at MIT and published in The Astrophysical Journal, was a precise measurement of the “accreting column” of hot gas above the white dwarf’s surface. “IXPE’s polarimetry allowed us to measure the height of this column to be almost 2,000 miles high, with fewer assumptions than past methods required,” explained lead author Sean Gunderson. The data also suggested the observed X-rays likely scattered off the white dwarf’s own surface, a detail crucial for modeling the system.
This breakthrough demonstrates how polarimetry can “see” the intricate details of cosmic accelerators. By revealing how matter behaves in the extreme magnetic environment of EX Hydrae, IXPE provides a new template for understanding the broader class of energetic binary systems, where white dwarfs, neutron stars, or black holes devour material from a stellar neighbor.
