Named ‘hypersoft X-ray sources,’ these objects give off very low-energy X-rays that are difficult to detect.
This composite image shows the locations of hypersoft X-ray sources in the face-on spiral galaxy Messier 101. Image credit: NASA / CXC / Univ. of Alabama / Muhibullah et al. / ESA / STScI / SAO / N. Wolk.
“We’ve never encountered a group of objects that act like this,” said Dr. Mustafa Muhibullah, an astronomer at the University of Alabama.
“Of course, the next step was to try to figure out what these things are.”
In their study, the astronomers analyzed archival data from NASA’s Chandra X-ray Observatory.
They found a total of 84 of hypersoft X-ray sources in the six different galaxies.
Two of the galaxies are spirals, the Andromeda galaxy and Messier 101, while the other four are ellipticals.
The hypersoft X-ray sources are located both in regions of active star formation and areas where there are older stars.
Despite their faintness in conventional X-ray bands, some of these sources are extraordinarily energetic.
The scientists believe hypersoft sources are not a single type of object but likely represent a mix of different systems, including white dwarfs pulling material from a companion star, the smoldering remnants of post-nova eruptions, and possibly even black holes accreting matter.
“Because low-energy X-rays border energetic ultraviolet radiation on the electromagnetic spectrum, we determined that these sources are producing large amounts of energetic ultraviolet radiation as well,” they said.
“It is unclear what types of objects are responsible for these low-energy X-rays and intense ultraviolet radiation.”
“We think they most likely involve a black hole, neutron star, or white dwarf pulling material from a companion star.”
The discovery suggests that there may be large populations of binary systems with energetic ultraviolet radiation that have been undetected until now.
“These clandestine X-ray sources are actually among the most energetic objects in galaxies, and they could be solving two cosmic mysteries at once,” Dr. Muhibullah said.
The discovery is reported in a paper published this month in the journal Nature Astronomy.
_____
M. Muhibullah et al. Hypersoft X-ray sources as a low-energy class of luminous cosmic emitter. Nat Astron, published online September 9, 2026; doi: 10.1038/s41550-026-02959-7
