A search of the Chandra archive found 84 hypersoft X-ray sources across six galaxies. These point-like objects radiate mainly below 0.3 keV and release most of their energy in easily absorbed extreme ultraviolet. Possible sources include accreting white dwarfs, neutron stars and black holes; some may offer new targets in searches for type Ia supernova progenitors and for radiation that ionises galactic gas.
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Eighty-four sources emerged across six galaxies
The peer-reviewed Nature Astronomy paper and the Chandra X-ray Center release report 84 hypersoft X-ray sources in archival data. The search covered six galaxies: M31 and M101 are spirals, while the other four are ellipticals. Sources appeared both in actively star-forming regions and in areas dominated by older stars. Their point-like, non-nuclear positions distinguish them from active galactic nuclei and link them instead to binary systems around individual stars.[1], [2]
Light below 0.3 keV escaped earlier surveys
The objects appeared in Chandra's lowest-energy images and vanished at higher energies; most of their X-ray output lies below 0.3 keV. Their spectra peak in the extreme ultraviolet, but interstellar hydrogen and helium readily absorb that light. The sources therefore occupy the band that X-ray telescopes find hardest to measure and are concealed by intervening gas where their ultraviolet output is strongest. A search focused on the archive's softest X-ray band opened the gap left by those two observational barriers.[1], [2]
One class may contain several kinds of binary
The researchers think hypersoft sources do not represent one kind of object. Binaries containing accreting white dwarfs, post-nova systems, neutron stars or black holes may all enter the same observational class. Some white-dwarf cases count among possible progenitors of type Ia supernovae, while intense extreme-ultraviolet output may contribute to ionising interstellar gas in certain galaxies. These ideas are not yet a definitive classification. The new catalogue identifies targets for separating which observed source corresponds to which physical system.[1], [2]
