NASA's Imaging X-ray Polarization Explorer (IXPE) has completed its inaugural X-ray polarization study of a white dwarf star, EX Hydrae. This investigation, conducted in 2024, utilized IXPE's capabilities to measure the height of an accreting gas column on the star to approximately 2,000 miles. The findings contribute to the understanding of the geometry and behavior of energetic binary systems.
Study Overview
IXPE observed EX Hydrae for approximately one week in 2024. The star system is located about 200 light-years from Earth within the constellation Hydra. The results of this study have been published in The Astrophysical Journal.
The research was led by astrophysics scientists from the Massachusetts Institute of Technology (MIT), with Sean Gunderson serving as the lead author. Co-authors and collaborating institutions included researchers from the University of Iowa, East Tennessee State University, University of Liége, and Embry Riddle Aeronautical University.
The IXPE mission is a collaborative effort between NASA and the Italian Space Agency, involving partners and science collaborators across 12 countries. NASA's Marshall Space Flight Center manages the mission, while BAE Systems, Inc., and the University of Colorado’s Laboratory for Atmospheric and Space Physics handle spacecraft operations.
Understanding EX Hydrae and White Dwarfs
A white dwarf star is a dense stellar remnant that forms after a star depletes its hydrogen fuel for fusion and is not massive enough to undergo a core-collapse supernova. These remnants typically have a diameter similar to Earth but a mass comparable to that of the Sun.
EX Hydrae exists as an intermediate polar binary system, which consists of a white dwarf and a main-sequence companion star. Gas from the companion star is continuously transferred onto the white dwarf, a process known as accretion. While EX Hydrae's magnetic field is strong enough to channel material towards its magnetic poles, it also allows for the formation of an accretion disk around the white dwarf, leading to its classification as an "intermediate polar."
During the accretion process, matter heats to temperatures reaching tens of millions of degrees Fahrenheit. This superheated material collides with other matter bound to the white dwarf, generating columns of gas that emit high-energy X-rays.
IXPE's Observational Capabilities and Key Findings
NASA’s IXPE is specifically designed to study the polarization of X-rays emitted by celestial objects. This capability allows researchers to determine the geometry of the objects producing the emissions, which are often too small for direct imaging.
The mission's X-ray polarization data enabled scientists to measure the height of the accreting gas column on EX Hydrae. This measurement indicated the column to be approximately 2,000 miles high. This determination was made with reduced reliance on assumptions compared to previous calculations. Sean Gunderson stated that the observed X-rays are believed to have scattered off the white dwarf's surface.
Broader Scientific Implications
The polarization data acquired from IXPE's study of EX Hydrae is expected to contribute to the scientific understanding of other highly energetic binary systems. This includes refining models of similar phenomena throughout the universe, especially those characterized by strong magnetic fields and X-ray emissions. The IXPE mission continues to explore other extreme objects such as black holes and neutron stars.