Chandra found 84 objects nobody had catalogued, and they may be supernovae waiting to go

Astronomers combing through two decades of public archive data from NASA’s Chandra X-ray Observatory have turned up 84 objects that behave unlike anything cataloged before, and the team says the discovery could help resolve two separate, long-standing puzzles in astrophysics at once. The objects give off unusually faint X-rays paired with intense ultraviolet radiation, a combination distinctive enough that researchers have given the class its own name: hypersoft X-ray sources.

A New Class Found Hiding in Six Galaxies

The team, led by Mustafa Muhibullah of the University of Alabama, searched Chandra archive images of six galaxies and found 84 hypersoft X-ray sources spread across them. Two of the galaxies, M31 and M101, are spirals better known as the Andromeda galaxy and the Pinwheel galaxy; the other four are ellipticals. The sources turned up both in regions of active star formation and in older stellar populations, suggesting whatever produces them is not confined to a single type of environment. The findings appear in a paper published in the journal Nature Astronomy.

Objects That Vanish at Higher Energies

Researchers identified the sources by a distinctive signature: they showed up clearly in Chandra images taken at the lowest X-ray energies but disappeared in images captured at higher energies, meaning they emit far more low-energy X-rays than high-energy ones, according to a NASA Science account of the discovery. Because low-energy X-rays sit right next to ultraviolet light on the electromagnetic spectrum, that pattern told the team the objects are also pumping out large amounts of energetic ultraviolet radiation, radiation intense enough to stand out even though it is notoriously difficult to observe directly, since interstellar hydrogen and helium gas readily absorb it before it can be detected.

Likely Binary Systems, Still Unidentified

The team’s leading explanation is that each hypersoft source is a binary system in which a black hole, neutron star, or white dwarf is pulling material off a companion star, heating that material until it radiates X-rays as it falls inward. Such binaries are already well known to astronomers, but never before with this particular combination of very soft X-rays and unusually bright ultraviolet output. Jimmy Irwin, a co-author on the study also at the University of Alabama, said the discovery suggests there may be sizable populations of these energetic binaries that have simply gone undetected until researchers knew what pattern to search for.

Muhibullah said the team’s first reaction on isolating the objects was confusion, since nothing in the existing catalog of X-ray binaries behaved quite the way these sources did, appearing bright at the very lowest energies Chandra can measure and then essentially disappearing at every higher energy band. That distinctive vanishing act is what eventually convinced the researchers they were looking at a genuinely new class of object rather than a set of previously known binaries caught in an unusual observing condition or a data-processing artifact.

A Possible Missing Link to Type Ia Supernovae

One of the two puzzles the discovery may help address involves Type Ia supernovae, stellar explosions that occur when a white dwarf pulling material from a companion crosses a critical mass threshold and detonates. Those explosions are essential tools for measuring the universe’s expansion, having helped establish that the expansion is accelerating, yet astronomers have never been able to directly catch the stars that turn into Type Ia supernovae before they explode. Irwin said finding a way to identify the progenitor systems before detonation “would be really important,” since researchers currently only get to study the supernovae after the explosion has already happened, leaving the ignition process itself poorly understood. Hypersoft sources, as accreting white-dwarf binaries, are plausible candidates for that missing progenitor population.

A Possible Answer for Stripped Interstellar Gas

The second mystery involves what strips electrons from the diffuse gas that fills the space between stars in some galaxies, a process that shapes how quickly new stars can form and influences the broader life cycle of a galaxy. Hot, massive stars contribute to that stripping but do not fully account for it, and the research team suggests the intense ultraviolet radiation pouring out of hypersoft X-ray sources could supply the missing piece. Rosanne Di Stefano of the Center for Astrophysics at Harvard and Smithsonian, another co-author, said mining the public Chandra archive let the team eliminate what had effectively been a blind spot in X-ray astronomy, since the combination of faint X-rays and absorbed ultraviolet light had kept these objects hidden even though the data needed to find them had been publicly available for years.

Why an Archive Search Found What Direct Observation Missed

Chandra has been operating since 1999, and its public archive now contains observations of thousands of galaxies collected over more than two decades, far more data than any single research team could analyze through fresh observing time alone. Because the hypersoft sources are faint specifically at low X-ray energies, a range many earlier studies filtered out or deprioritized while searching for brighter, more conventional X-ray binaries, the objects had effectively been sitting in plain sight in existing datasets. The Alabama-led team’s contribution was less about pointing Chandra at something new and more about asking a question nobody had systematically asked of the old data: what happens if researchers specifically isolate the faintest, softest X-ray signals and see what survives. NASA’s Marshall Space Flight Center manages the Chandra program, while the Smithsonian Astrophysical Observatory’s Chandra X-ray Center in Cambridge, Massachusetts, oversees science operations.

This article was created with the assistance of AI and reviewed by an editor.

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