Scientists get a look at the innermost region of a white dwarf for the first time

NASA’s IXPE (Imaging X-ray Polarization Explorer) focused on a system called the intermediate polar EX Hydrae in 2024 for nearly one week. EX Hydrae is a white dwarf system located about 200 light-years from Earth. A white dwarf is the dense core that remains after a Sun-like star dies.

EX Hydrae exists in a binary system with a main-sequence companion star. EX Hydrae has a magnetic field of intermediate strength, which pulls material from this star. A strong magnetic field pulls material from the companion star into the white dwarf’s magnetic poles. Where the magnetic field is very weak, the stellar material swirls around the dwarf in an accretion disk. In EX Hydrae, stellar material forms an accretion disk and also gets pulled towards the white dwarf’s poles. This is why it is classified as an intermediate polar.

From IXPE’s observation, astronomers were able to measure the polarization of EX Hydrae. The measurements revealed an eight percent polarization degree. This was much higher than some models had predicted.

IXPE’s data also revealed the source of the X-rays. The X-rays were found to be coming from a column that channels super-hot gas from the inner disk onto the dwarf’s surface. The astronomers estimated this column to be about 2,000 miles high — much higher than predicted.

The direction of EX Hydrae’s X-ray polarization was not left out of the study. The team measured the polarization direction of the X-rays and discovered it was perpendicular to the column of incoming gas. This suggested that the X-rays emitted by the column reflected off the white dwarf’s surface before scattering into space and reaching IXPE.

The team published the study in the Astrophysical Journal. They plan to use X-ray polarization to study more white dwarf systems. This could help scientists gain more understanding of bigger-scale cosmic events that involve similar physics.