The faintest X-rays may be hiding the stars that explode
A search through NASA’s Chandra archive found 84 previously unseen hypersoft X-ray sources whose possible ultraviolet output could clarify the origins of Type Ia supernovae.
A signal that disappears when astronomers look harder
The sources showed up in Chandra images at the lowest X-ray energies, then vanished from higher-energy views. That is the opposite of the harder, brighter X-ray signature astronomers often use to identify violent cosmic systems. Their low-energy X-rays sit next to ultraviolet on the electromagnetic spectrum, so the team concludes they are likely producing intense ultraviolet radiation too. science
The objects were found in the spiral galaxies M101, also called the Pinwheel, and M31, the Andromeda galaxy, plus four elliptical galaxies. Seven of the 84 are in M101, about 21 million light-years away. They occur both where stars are actively forming and in older stellar neighborhoods. science +1
The leading explanation is still a guess
Researchers do not yet know what the sources are. Their best explanation is a binary system: a black hole, neutron star or white dwarf pulling material from a companion star. That material heats up and emits X-rays before falling onto the compact object, but known binaries have not shown this combination of unusually soft X-rays and bright ultraviolet light. science +1
The discovery could mean that astronomers have missed a large population of such binaries. They were hard to find because low-energy X-rays require long observations, while the ultraviolet light is absorbed by the hydrogen and helium gas between stars. Chandra’s archive made the faint pattern searchable across old observations. science
Why Type Ia supernovae are part of the story
Some white-dwarf binaries may eventually detonate as Type Ia supernovae, the explosions astronomers use to measure cosmic expansion. Scientists have long searched for the systems that precede them, without identifying a definitive route from binary to blast. The new sources are candidates for that missing population—not proof that any one of them will explode. science +1
The same ultraviolet output may help explain how gas between stars loses electrons, a process that affects star formation and galaxy evolution. The next step is identification: follow-up observations must determine whether one kind of binary, or several, produces the hypersoft signature. science