Astronomers may have spotted the afterglow from an epic cosmic event known as a "kilonova."

Kilonovas occur after the collision of two hyper-dense neutron stars, which are the remnants of stars that have died in supernova explosions. Astronomers think they have spotted an afterglow in X-rays from the event, which is dubbed GW170817.

The discovery team suggests that as the debris expanded out from the collision, the resulting sonic-boom-like shock heated up surrounding materials. The heating generated X-rays. 

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An artist's conception illustrates the aftermath of a "kilonova," a powerful event that happens when two neutron stars merge. (Image credit: NASA/CXC/M. Weiss)

Alternatively, however, a similar effect may be produced due to materials falling towards a black hole caused by the neutron star merger, so astronomers caution the finding is tentative. Either type of find, however, would be the first known to science.

"We have entered uncharted territory here in studying the aftermath of a neutron star merger," lead researcher Aprajita Hajela, an astrophysics graduate student at Northwestern University, said in a statement. "We are looking at something new and extraordinary for the very first time. This gives us an opportunity to study and understand new physical processes, which have not before been observed."

A 3D visualization of a neutron star. (Image credit: Shutterstock)

The neutron star event was already known to science, following an Aug. 17, 2017 merger that produced the first-ever detection of such an event by gravitational waves, or ripples of space-time. Astronomers continue to study the region to find out how the area is evolving.

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Astronomers spotted X-ray emissions soon after the event, using NASA's Chandra X-ray Observatory, but the emission began to fade in early 2018. Hajela's team, however, showed the decline in brightness stopped in 2020, with the X-ray emission remaining nearly constant. 

The consistency in X-ray brightness was what pointed to this being an unusual event, team members said. "A completely different source of X-rays appears to be needed to explain what we’re seeing," Raffaella Margutti, an astrophysicist at the University of California at Berkeley and a senior author of the study, said in the same statement.

Figuring out what the ultimate cause was, however, will require more follow-up study. If it is indeed a kilonova, the researchers expect to see the X-ray and radio emissions get brighter as the shock continues to plow through the nearby environment. But if it is a black hole, the output should decline or remain steady.

A study based on the research was published Monday (Feb. 28) in The Astrophysical Journal Letters.

Follow Elizabeth Howell on Twitter @howellspace. Follow us on Twitter @Spacedotcom or Facebook.