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Astronomers using NASA Chandra, the Chandra X-ray Observatory, have identified a previously unrecognized class of cosmic sources with an unusual combination of extremely low-energy X-rays and strong ultraviolet radiation.
Called hypersoft X-ray sources (HSSs), the objects were discovered by analysing publicly available Chandra observations. Researchers identified 84 sources across six galaxies, with the findings published in Nature Astronomy on September 9, 2026.
The discovery could provide new clues about compact stellar systems and potentially help astronomers investigate questions surrounding Type Ia supernova progenitors and the ionization of gas within galaxies.
What Did NASA Chandra Discover?
The researchers searched Chandra observations for objects that appeared in the telescope’s lowest-energy X-ray images but disappeared at higher energies.
The sources are detected primarily or exclusively below 0.3 keV, while their spectra appear likely to peak in the extreme-ultraviolet (EUV) region. This unusual combination distinguishes them from many familiar X-ray sources.
The 84 objects were found in six galaxies, including M31 (the Andromeda Galaxy) and M101 (the Pinwheel Galaxy), along with four elliptical galaxies.
| Discovery | Details |
| Object class | Hypersoft X-ray sources |
| Number identified | 84 |
| Galaxies | 6 |
| Observatory | NASA Chandra |
| X-ray energy | Primarily below 0.3 keV |
| Additional emission | Likely extreme ultraviolet |
| Research published | Nature Astronomy, September 9, 2026 |
Why Were They Difficult to Detect?
The unusual radiation profile may explain why these sources escaped detection in earlier observations.
Their X-rays are concentrated at very low energies, making them challenging to identify. Meanwhile, EUV radiation can be absorbed by hydrogen and helium gas between stars. This combination creates an observational challenge for astronomers.
By searching archival Chandra data specifically for sources that disappeared at higher X-ray energies, researchers were able to identify this previously overlooked population.
What Could Hypersoft X-Ray Sources Be?
The exact nature of the objects remains unknown.
Researchers propose that hypersoft X-ray sources may represent X-ray binaries spanning several physical classes. Such systems can contain a compact object that accretes material from a companion star.
Possible compact objects include:
- White dwarfs
- Neutron stars
- Black holes
However, scientists have not established that all 84 sources belong to one particular type. The population could contain multiple physical classes.
Could They Be Connected to Type Ia Supernovae?
One of the most interesting possibilities involves accreting white dwarfs.
If some hypersoft sources are confirmed to be accreting white-dwarf systems, they could provide new candidates for studying potential Type Ia supernova progenitors. These powerful explosions are important in astronomy because they can be used to measure cosmic distances.
However, the newly discovered sources have not been confirmed as future Type Ia supernovae. The connection remains a possibility that requires further investigation.
Could They Help Explain Galactic Gas Ionization?
The discovery may also have implications for the gas between stars.
Energetic ultraviolet radiation can remove electrons from atoms, a process known as ionization. Because hypersoft sources appear to produce substantial EUV emission, researchers suggest they may contribute to the ionization of gas in galaxies.
This is not yet a solved mystery. Further observations are needed to determine how significant these sources are compared with other sources of ionizing radiation.
What Happens Next?
The next challenge is determining exactly what the 84 objects are.
Astronomers can use follow-up observations to study their variability, environments and radiation at other wavelengths. These observations could reveal whether hypersoft sources represent one physical phenomenon or several types of compact stellar systems.
Researchers will also be interested in determining how many of the sources involve accreting white dwarfs and whether they could provide useful candidates for studying possible Type Ia supernova progenitors.
Frequently Asked Questions
What did NASA Chandra discover?
NASA Chandra observations helped researchers identify 84 hypersoft X-ray sources across six galaxies. They produce unusually low-energy X-rays and are thought to emit substantial extreme-ultraviolet radiation.
What are hypersoft X-ray sources?
They are a newly identified class of luminous, point-like, non-nuclear X-ray sources detected primarily or exclusively at very low X-ray energies.
What could these sources be?
Researchers propose that they may represent different types of X-ray binaries involving accreting white dwarfs, black holes or other compact objects. Their exact nature remains uncertain.
Could they become Type Ia supernovae?
Some could potentially be associated with accreting white-dwarf systems that are candidates for Type Ia supernova progenitors. This connection has not yet been confirmed.
Why were they difficult to detect?
Their very low-energy X-rays are difficult to observe, while their extreme-ultraviolet radiation can be absorbed by hydrogen and helium gas between stars.
Conclusion
The discovery of 84 hypersoft X-ray sources gives astronomers a new population of unusual cosmic objects to study. Using NASA Chandra, researchers identified sources that emit primarily at very low X-ray energies and may produce substantial extreme-ultraviolet radiation.
The objects could provide new clues about accreting white dwarfs, neutron stars and black holes. They may also help researchers investigate potential Type Ia supernova progenitors and the sources responsible for ionizing gas within galaxies.
For now, their true nature remains unresolved. NASA Chandra has revealed the population; future observations will determine what these mysterious sources actually are.