Chandra observations reveal a previously overlooked population that could offer clues to stellar explosions and the radiation shaping galaxies.
Astronomers using NASA’s Chandra X-ray Observatory have identified 84 unusual sources across six galaxies, uncovering a population whose radiation differs sharply from familiar X-ray-emitting systems.
Called “hypersoft X-ray sources,” the objects appear at the lowest X-ray energies detectable by Chandra but produce little or no detectable emission at higher energies. The discovery, announced on September 9 and published in Nature Astronomy, emerged from a search through the observatory’s publicly available archive.
The sources occur in both spiral and elliptical galaxies, including Andromeda and the Pinwheel Galaxy. Researchers found them in environments containing young stars as well as older stellar populations, suggesting the phenomenon extends across different galactic settings.
Their unusual radiation offers a way to investigate a largely hidden part of the universe. Extreme-ultraviolet light is readily absorbed by hydrogen and helium between stars, making distant sources difficult to observe directly. The team instead searched for faint X-rays that could reveal objects emitting much of their energy in that obscured wavelength range.
The distinction matters: the strong extreme-ultraviolet output is inferred from the sources’ unusually soft X-ray emission, rather than directly observed across the entire population. “Soft” describes the comparatively low energy of the X-ray photons, rather than an object’s overall brightness.
Researchers suspect that many sources involve compact stellar remnants drawing material from companion stars. White dwarfs are leading candidates, while black holes or neutron stars could account for others. These types of systems are already known; what distinguishes the new group is its unusual radiation pattern.
Several of the less luminous sources in Andromeda are associated with novae—outbursts caused by material accumulating on white dwarfs. Other sources could involve sustained nuclear burning on a white dwarf whose expanded outer layers shift its emission toward extreme-ultraviolet wavelengths.
If that explanation holds, some objects might represent an overlooked stage preceding Type Ia supernovae, the stellar explosions astronomers use to study cosmic expansion. Their ultraviolet radiation could also help explain how electrons are stripped from gas between stars, influencing the conditions in which galaxies form new stars. Both possibilities remain under investigation.
The brightest sources present a further challenge: a white-dwarf explanation may be insufficient, while ordinary feeding black holes should produce more energetic X-rays than researchers observe. More than one physical mechanism may therefore lie behind the new classification.
For now, the discovery establishes a distinctive population to investigate. Determining what powers it could clarify processes that existing observations have struggled to reveal.
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