Astronomers have, for the first time, obtained a high-resolution view of a white dwarf star actively siphoning material from a nearby companion. Using NASA’s Imaging X-ray Polarimetry Explorer (IXPE), a team from MIT observed the system EX Hydrae, located 200 light-years from Earth, revealing details about the violent interaction between these stars. This discovery provides critical insights into the behavior of “intermediate polar” binaries – systems where a dead star aggressively consumes its neighbor.
The Dynamics of Stellar Cannibalism
EX Hydrae consists of a white dwarf, the dense remnant of a sun-like star, and its victim, which orbits it every 98 minutes. The white dwarf’s magnetic field pulls material from the companion star, creating a column of superheated gas roughly 2,000 miles (3,200 kilometers) tall. This column rains down onto the white dwarf, releasing intense X-ray radiation.
This scale is significant : the column is almost half the radius of the white dwarf itself, meaning a substantial portion of the companion star’s material is being directly consumed. The team also confirmed that X-rays are bouncing off the white dwarf’s surface before being scattered, a phenomenon predicted but never before observed directly.
Why This Matters: Accretion Disks and Magnetic Fields
White dwarfs are at the end of stellar evolution and can take two routes to destruction; either by exploding into a Type Ia supernova or by slowly cooling into a black dwarf. The key difference between these two paths lies in how the white dwarf acquires mass from its companion star.
Intermediate polars, like EX Hydrae, fall into a unique category where the magnetic field isn’t strong enough to form a complete accretion disk (a swirling mass of stolen material) around the white dwarf, but it is strong enough to funnel material towards the poles. This process creates an “accretion curtain”—a downpour of stellar matter at millions of miles per hour. The collision of infalling material creates turbulent, multi-million-degree gas columns that emit detectable X-rays.
The Power of X-ray Polarimetry
The IXPE spacecraft measured the polarization of X-rays emitted from EX Hydrae, which revealed a surprisingly high degree of polarization (8%). This means the waves of radiation were aligned in a specific direction, confirming the presence of the magnetic field and the intense energy flow.
“We showed that X-ray polarimetry can be used to make detailed measurements of the white dwarf’s accretion geometry,” said Sean Gunderson, team leader at MIT.
This technique provides a clearer picture of the extreme environment around the white dwarf than traditional telescopes, which often only see a faint dot in the sky. By analyzing the polarization, scientists can reconstruct the innermost, most energetic region of the system.
Future Implications: Understanding Supernovae
The research team plans to expand their investigation to other white dwarf systems, aiming to better understand how these stars eventually explode as Type Ia supernovae. These supernovae are crucial for measuring cosmic distances, so understanding the factors leading to their explosions is vital for mapping the universe.
The study of stellar vampires offers a direct glimpse into the violent final stages of star systems, shedding light on both their immediate behavior and their ultimate fate.
