Stellar Saturdays · the week's best pictures from off-world

From The Decisional Pass

Here's a galaxy with seven things in it that nobody should have been able to find. The good part: nobody took a new picture to find them. The data had been sitting in a public archive for more than twenty years.

🔭 Eighty-four of something new

On September 9, NASA and the Chandra X-ray Center announced 84 objects that don't fit anything on the books. They're calling them hypersoft X-ray sources. Seven of them are in the galaxy above, M101, the Pinwheel, about 21 million light-years out. The rest are spread across five more galaxies: Andromeda, and four ellipticals.

The best guess at what they are is a binary. Something dense, a black hole or a neutron star or a white dwarf, pulling gas off a companion star. The gas piles up, heats up, and glows on the way down. That part is ordinary, and we have catalogues full of it. What isn't ordinary is the light that comes out: almost none of it lands where an X-ray telescope is actually any good at looking.

The paper is in Nature Astronomy, led by Mustafa Muhibullah at the University of Alabama with Jimmy Irwin, alongside Rosanne DiStefano at the Center for Astrophysics. Muhibullah's line about them is the one I'd keep: "We've never encountered a group of objects that act like this."

🟣 Why the picture looks so empty

Strip the optical layer away and here is what Chandra actually had to work with.

That's the whole problem in one frame. These sources put out almost all their X-rays below 0.3 keV, at the very bottom edge of what an X-ray telescope can register. Go looking with a standard survey cut and they simply aren't in your results, because the energies they emit at are the ones your instrument is worst at.

And the obvious workaround doesn't work either. Most of the energy is actually coming out in the extreme ultraviolet, which would be easy to spot except that hydrogen and helium in intergalactic space soak it up before it gets here. So the objects are loud in a band we can't see through, and quiet in the band we can.

Here's the bit that got me, as somebody who has spent a lot of time on old data. This wasn't a new observing campaign. Nobody got telescope time, nobody waited on a scheduler. It's 274 hours of M101, 25 separate Chandra pointings taken between March 2000 and January 2005, re-examined with the question asked differently. The photons were always there, recorded and public the whole time. The catalogue cut was throwing them away before a human ever saw them.

💡 Two things this might explain

The reason to care beyond the picture is that these objects sit on top of two open problems.

The first is Type Ia supernovae. When a white dwarf in a binary takes on enough mass, it detonates, and it does so at a predictable brightness. That predictability is what we use as a distance ladder to measure how fast the universe is expanding. It is slightly embarrassing that we've never firmly identified the systems that go on to do it. A population of white dwarfs quietly eating their companions is a plausible place to look.

The second is the intergalactic medium. The gas between galaxies is ionised, its electrons stripped off, and the sums for what did the stripping have never quite closed. Objects pouring out extreme ultraviolet are exactly the kind of thing that would do it, and they'd change how quickly gas can cool and fall into a galaxy to make stars.

Both of those are "maybe" at this stage, and worth saying so plainly. Eighty-four objects across six galaxies is a population, not a proof, and the identifications are inferences drawn from spectra rather than something anyone has watched happen. What it does establish is that there is a class of object sitting in twenty-five years of public archive that nobody had pulled out. Which raises a question worth sitting with: what else is in there, filed under nothing?

📚 References

• NASA's Chandra Unveils Mysterious X-Ray Objects — NASA Science, September 9, 2026
• M101: NASA's Chandra Unveils Mysterious X-ray Objects — Chandra X-ray Observatory, September 9, 2026
• A Hidden Population of Cosmic Beacons: Discovering Hypersoft X-ray Sources with Chandra — Chandra Blog, September 2026

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