Last Year's Neutron Star Collision Probably Created a Black Hole, Scientists Say

Scientists spotted two superdense stars crash into each other for the first time ever back in August, and they've been fascinated by the incident since. Now, they think they know what was produced by the enormous collision: a black hole—and not just any black hole, but the smallest one scientists have ever studied.

That's according to a new paper posted on the pre-print site and scheduled for publication in The Astrophysical Journal Letters.

The event, which scientists dubbed GW170817 for the date on which it occurred, has been the subject of many studies since the collision and its accompanying burst of light were first announced in October.

"GW170817 is the astronomical event that keeps on giving," co-author J. Craig Wheeler, a physicist the University of Texas, said in a press release. "We are learning so much about the astrophysics of the densest known objects [neutron stars] from this one event."

The August collision wasn't the first successful detection of gravitational waves, the faint ripples in the universe produced by the movement of extremely rapid objects. But previous detections have exclusively involved black holes, which produce only gravitational waves and no visible light. Last year's collision was the first in which scientists could actually see what was happening during and after the collision.

The scientists estimate that the collision produced something with about 2.7 times the mass of our sun. But that happens to be right in the sticky area between very large neutron star and very small black hole.

So the new paper looks at x-rays using data from NASA's Chandra X-Ray Observatory, a space-based telescope. In particular, they looked at what was happening almost four months after the initial collision. All the light they saw could have been caused by the impact itself, which would have produced a giant shockwave. The scientists didn't see any signal that pointed to a new neutron star hiding within the rubble—and that makes them suspect the collision produced a black hole instead.

An artist's depiction of the neutron star collision scientists observed in August, which they now believe created a small black hole. NASA/CXC/M.Weiss

"We may have answered one of the most basic questions about this dazzling event: what did it make?" co-author Pawan Kumar, an astrophysicist at the University of Texas at Austin, said in a press release. "Astronomers have long suspected that neutron star mergers would form a black hole and produce bursts of radiation, but we lacked a strong case for it until now."

That said, it's not a done deal and the scientists want to keep their eye on the collision's leftovers for a little while longer. If it's a neutron star after all, it should start acting up over the next year or two, producing more and more x-rays. If Chandra doesn't see any such activity, that should seal the deal: it's a tiny black hole.