Northwestern astrophysicists find possibly one of the lightest black holes ever

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Northwestern University astrophysicists have helped discover a neutron star crashing into a mysterious object, which they think is one of the lightest black holes ever seen. This would change scientists' prior beliefs about what kinds of astronomical entities exist.

"One of the exciting things is because we saw one of these, it's likely that there are many more out there, and we're likely to see more in the future," said Michael Zevin, an astrophysicist at the Adler Planetarium and a Northwestern visiting scholar.

Neutron stars form when a massive star runs out of fuel and collapses. The more massive the original star, the more likely it will collapse into something even denser: a black hole.

The mysterious object was in the "mass gap," or the range between the heaviest neutron star and the lightest black hole. For 20 years, astronomers were unsure if objects existed in this range.

But the supposed black hole is "sitting right" in the mass gap, Zevin said.

From what researchers have seen, neutron stars reach about two times the mass of the sun, and black holes are greater than five times the mass, he said.

Michael Zevin helped lead the scientific paper that detailed the discovery of the mysterious object in the so-called "mass gap."

Courtesy of Northwestern University

Researchers are not "entirely positive" the mysterious object is a black hole, though it "most probably" is, he said.

This supposed black hole was "very, very light," Zevin said. It was three to four times as massive as the sun, whereas most black holes detected in this galaxy are 10 to 20 times as massive.

The LIGO-Virgo-KAGRA collaboration, which involves over 1,000 scientists around the world, first identified the mysterious object in May 2023. Afterward, Zevin and Northwestern postdoctoral fellow Sylvia Biscoveanu led the scientific paper, analyzing the implications of the discovery.

Researchers used gravitational wave detectors to find the merging. Space is like malleable fabric, Zevin said, which moves and distorts when it encounters matter and energy.

So gravitational waves are like "ripples in the fabric of space time," he said.

The detectors are "incredibly precise rulers measuring what distances stretch and squeeze by an infinitesimal amount," Zevin said.

Biscoveanu said all gravitational wave events detected so far are from binary systems of two black holes, two neutron stars or one neutron star and one black hole.

The existence of objects in the mass gap could affect science's understanding of phenomena like supernova explosions.

"If we're actually finding with gravitational waves [that] this mass gap doesn't exist, theorists are going to have to go back and reevaluate the sorts of models that they've been using," Biscoveanu said.

Sylvia Biscoveanu said more mergers between neutron stars and black holes likely exist, and researchers want to see more of them.

Courtesy of Northwestern University

Biscoveanu said she sees two outcomes after the collision. The neutron star could get "swallowed" by the black hole, or it could get "shredded apart" before it enters the black hole.

"It's just a matter of, you know, in the final moments, they are close enough together that one of these two things would happen," Biscoveanu said.

If the neutron star gets shredded, material that made up the neutron star will form a "little disc" around the black hole, she said.

"You can think of it like how Saturn has a ring. It would be like a ring of material that forms around the black hole. It's not necessarily stable, like planetary rings," Biscoveanu said.

Researchers hope to see more mergers between neutron stars and black holes. Scientists are learning these phenomena, consisting of a neutron star and a black hole, can be accompanied by electromagnetic radiation, or visible light, she said.

"There's others out there," Biscoveanu said. "So it could be observed not just in gravitational waves but also using regular telescopes via the light that's emitted."

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