Chandra archive reveals 84 unexplained low-energy X-ray sources

Astronomers combing through public archive data from NASA's Chandra X-ray Observatory have found 84 objects that behave unlike anything catalogued before. The objects give off unusually low-energy X-rays alongside what researchers infer to be intense ultraviolet radiation, a combination that has not been documented as a class until now. The find is described in a paper published in Nature Astronomy and led by Mustafa Muhibullah of the University of Alabama.

The team, which also includes Jimmy Irwin of the University of Alabama and Rosanne Di Stefano of the Center for Astrophysics | Harvard & Smithsonian, searched six galaxies for these sources. Two are spiral galaxies, M31 (the Andromeda galaxy) and M101 (the Pinwheel galaxy), and the other four are elliptical galaxies. The objects turned up both in regions of active star formation and in areas containing older stars, and the researchers have named them "hypersoft X-ray sources" because of how low-energy their X-ray emission is.

"We've never encountered a group of objects that act like this," Muhibullah said. "Of course, the next step was to try to figure out what these things are." According to Scientific American's account of the work, the underlying signal had been sitting in Chandra data collected between 1999 and 2017, but scientists had largely dismissed it as noise because it fell in the telescope's lowest-energy channels.

The researchers found the objects by comparing Chandra images taken at different X-ray energies. Each hypersoft source showed up clearly in images built from the lowest-energy X-rays but disappeared in images built from higher-energy X-rays, meaning it emits far more low-energy X-rays than high-energy ones. Because low-energy X-rays border energetic ultraviolet radiation on the electromagnetic spectrum, the researchers determined that these sources are producing large amounts of energetic ultraviolet radiation as well, even though the ultraviolet emission itself has not been directly observed.

What kind of object produces this signature is not settled. The team's leading idea is a binary system: a black hole, neutron star, or white dwarf pulling material off a companion star, with the material heating up as it falls and giving off X-rays before it lands on the compact object. Such binary systems have been observed before, but never with X-rays this low in energy paired with ultraviolet radiation this bright, according to Muhibullah and Irwin. Di Stefano and Irwin, quoted separately in Scientific American, both stressed that hypersoft may not describe one type of object at all — Irwin said the sources are "probably a heterogeneous mix of different types of objects," with the hypersoft label describing an energy signature rather than a single physical class.

The reason these objects went unnoticed for so long comes down to two problems. Low-energy X-rays are difficult for X-ray telescopes to detect in the first place, and the researchers say the stronger ultraviolet radiation these objects likely produce is largely absorbed by hydrogen and helium gas that fills the space between stars, forming what one of the researchers described as a nearly impenetrable barrier. "By combing through the Chandra archive, we were able to eliminate what used to be a blind spot for telescopes," said Di Stefano.

The team suggests these objects could bear on two separate open questions in astrophysics. The first concerns Type Ia supernovae, explosions thought to occur when a white dwarf pulling material from a companion star eventually detonates. These supernovae are important because they have been used to measure the expansion of the universe, including the discovery that the expansion is accelerating, but astronomers have long searched without success for the specific systems that go on to explode this way. "If we could find a way to spot these Type Ia supernova explosions before they go off, that would be really important," said Irwin. "Right now, we study them after they've exploded, and astronomers have struggled to understand what is actually ignited."

The second puzzle involves what strips electrons from gas that lies between stars in some galaxies, a process that can affect how quickly new stars form and shape the broader life cycle of a galaxy. Hot, massive stars are known to contribute to this stripping, but they do not fully account for it. The intense ultraviolet radiation the researchers infer from hypersoft sources may supply some of the missing effect. Scientific American notes that some galaxies show this ionized gas even though they lack the central black holes usually credited with producing it, leaving hypersoft sources as one possible explanation for the gap.

Muhibullah described the sources as "among the most energetic objects in galaxies," and said they "could be solving two cosmic mysteries at once." Di Stefano offered a more cautious framing, calling the 84 detected sources "the tip of the iceberg": because most of the ultraviolet light these objects likely emit is absorbed before it can escape a galaxy, each detected source may represent many more that remain hidden. Irwin made a similar point, saying that with a telescope sensitive to extreme ultraviolet light and no absorbing gas in the way, these sources "would be booming bright in these galaxies, and they would not have been missed."

The nature of individual hypersoft sources is still unresolved. The researchers state plainly that it is unclear what types of objects are responsible for the low-energy X-rays and inferred ultraviolet radiation, and their proposed mechanism — a compact object accreting material from a companion star — remains their best explanation rather than a confirmed identification. The ultraviolet emission itself has not been directly detected; it is inferred from the shape of the X-ray spectrum and the physics of where low-energy X-rays sit relative to ultraviolet light on the electromagnetic spectrum.

The discovery does suggest that a large population of similar binary systems, glowing with energetic ultraviolet radiation, has gone undetected until now. But confirming that hypersoft sources actually explain either the source of Type Ia supernovae or the electron-stripping of interstellar gas will require further work beyond identifying the 84 candidates found so far.