In Old NASA Data, Scientists Find a Surprising New Type of Cosmic Object

Even within our cosmic neighborhood, the unexpected still turns up.

The latest surprise is a new class of X-ray beacons in nearby galaxies, a discovery that could prove key to solving some astronomical mysteries.

“They are very luminous,” said Mustafa Muhibullah, a graduate student at the University of Alabama who led the research. “They are numerous in galaxies.”

Some of these newly discovered objects could be precursors of certain types of exploding stars, or supernovas. Others could be exotic combinations like a triple-star system in which two of the stars have collapsed into black holes.

Remarkably, the finding had sat, waiting to be discovered, in data collected by NASA’s Chandra X-ray Observatory between 1999 and 2017.

Jimmy Irwin, a professor of physics and astronomy at the University of Alabama, started looking at that data about a dozen years ago.

X-rays are a form of light, but at shorter wavelengths than ultraviolet light and the visible-wavelength colors that we see. Scientists call higher-energy X-rays “hard” and ones at the lower end of the spectrum “soft.”

Chandra collects X-rays over a wide range of energies, and astronomers had generally ignored the softest X-rays because the detectors are not well calibrated to detect the few photons in this range.

“Nearly everyone kind of universally chops off the very-low-energy channels and says, ‘I don’t care about them,’” Dr. Irwin said. “I thought, ‘Why not look?’”

He found known X-ray sources like neutron stars and black holes, but he also spotted some that were emitting X-rays at only the lowest energies.

Rosanne Di Stefano, of the Center for Astrophysics in Cambridge, Mass., said she pushed Dr. Irwin to investigate further. She suspected that these were very bright ultraviolet objects that astronomers couldn’t see because the shortest, or extreme, ultraviolet wavelengths are blocked by hydrogen and helium gas between stars.

But if they also emit some soft X-rays, that could provide a way around this blind spot.

Dr. Irwin assigned the project to Mr. Muhibullah, one of his graduate students. Searches of six nearby galaxies yielded 84 objects that primarily emitted these lowest-energy X-rays. Almost all of them had not been previously known.

Mr. Muhibullah, Dr. Irwin and Dr. Di Stefano described their findings in a paper published last week in the journal Nature Astronomy.

Astronomers had already classified some celestial objects that emitted low-energy X-rays as “supersoft.” The new objects emit just the lowest-energy slice of supersoft X-rays, so the researchers call them “hypersoft.”

Mr. Muhibullah said they had since examined four additional galaxies, pushing the total number of hypersoft X-ray sources to 145.

“They’re not insignificant, rare oddities that are hard to find,” Dr. Irwin said.

Next question: What are these things?

For Dr. Di Stefano, the possible answers seem to address a couple of problems.

Some of the previously known supersoft X-ray sources were disappearing and reappearing. The best model for this, she said, was a type of white dwarf star, the collapsed core of a star that is running out of fuel to sustain its nuclear reactions.

If a white dwarf expanded and cooled, its light could shift to the longer wavelengths of the extreme ultraviolet and hypersoft X-rays and effectively vanish.

“That’s why I was so excited when Jimmy first told me about it, because it seemed like this was a missing class of objects,” Dr. Di Stefano said.

A large population of these cooler, hard-to-see white dwarfs could, in turn, help solve two other mysteries.

Explosions known as Type 1a supernovas blow up in a uniform manner, which allows them to be used as so-called “standard candles” for measuring cosmic distances.

Type 1a supernovas are thought to occur when a white dwarf gains weight beyond a certain threshold, setting off a runaway thermonuclear outburst. But astronomers have struggled to find enough objects that could explain the number of Type 1a supernovas that they see.

The new hypersoft X-ray objects could also explain how helium and heavier elements become ionized in certain galaxies.

Other theorists have come up with additional possibilities as to what the hypersoft X-ray sources could be.

When Sergei Popov of the Sternberg Astronomical Institute in Russia and Galina Lipunova of the Max Planck Institute for Radio Astronomy in Germany saw a preprint of the paper in February, they started brainstorming what could emit hypersoft X-rays but not hard X-rays.

“That’s the main fun in doing astrophysics, to read about new discoveries of some enigmatic sources, and to think, ‘What the hell this can be?’” Dr. Popov said.

If material from a companion star falls into a single black hole, it will generate hard X-rays, which would have been easily observed.

But for two black holes orbiting each other, very little material from a third star in the system would fall into either. Instead, the material would accumulate in a hot disk of debris that glows in extreme ultraviolet and hypersoft X-ray light.

Dr. Lipunova acknowledged that it would not be an easy model to test — unless the two black holes merged and unleashed a high-energy event known as a gamma-ray burst.

“If one of these sources would vanish, and the gamma-ray burst appears instead in this point of the sky, that would be a brilliant event to prove the model,” she said.