When two neutron stars sidle up to each other, sparks are bound to fly—well, more like a super bright collision. And in 2017, one particular neutron star merger left behind something unusual in its wake that the Chandra X-ray Observatory picked up as stray X-rays. No one knows for sure what these mysterious X-rays mean, but researchers from Northwestern University and the University of California, Berkeley think they’re close to identifying the source

Neutron stars are some of the densest objects in our universe and occur when the core of a huge star collapses. And when these super dense, dead stars get too close, it creates an immense burst of light, called a kilonova. Kilonovas are about 1000 times brighter than novas, which are caused by the eruption of a white dwarf. But kilonovas are about 1/10th to 1/100th the brightness of the more famous, showy star explosions, supernovas. The activity during a kilonova also forges a myriad of radioactive elements heavier than iron. So it turns out you can create gold—if you have two colliding neutron stars on hand. As they decay, these radioactive elements emit light.  

In August 2017, two neutron stars created a kilonova thot shot off a jet of high-energy particles. Cataloged as GW170817, this was the first kilonova scientists have detected with both light and gravitational waves. Three and a half years after the collision, the jet has faded away, but remnants of the epic cosmic event were left behind and seen as peculiar X-rays. 

[Related: This ‘kilonova’ shines so bright, it defies the odds]

The group of researchers from Northwestern University and University of California, Berkeley propose a couple potential sources for the remaining ghostly X-rays, which they write in a paper accepted by the Astrophysical Journal Letters that’s pending publication. 

One theory is that the rays might be the result of a phenomenon called a kilonova afterglow. This occurs when debris from the neutron star collision moves so fast that it creates a sort of extragalactic “sonic boom,” according to Aprajita Hajela, the lead author of the upcoming study and a doctoral candidate at Northwestern. 

“That shock heats up the material … and that material then introduces this kilonova afterglow,” Hajela says. The hot leftover material could then emit the X-rays scientists observed. 

The second theory is that a black hole was produced in the collision, and as the material fell into the void, it emitted the mysterious X-rays, Hajela explains. To find out which theory holds, the researchers are using Chandra and radio telescopes to look for radio emissions. If the site of the collision starts producing radio waves and the X-rays get brighter, then it could be a kilonova afterglow. If no radio waves show up and the X-rays start to decay, then it could be a black hole, according to Hajela.

The researchers can’t say yet which scenario is more likely, but they note the rarity of the observation. Neither a kilonova afterglow nor a black hole produced by a kilonova has ever been observed before. 

“Both of these scenarios are equally very exciting,” Hajela says. “This will definitely tell us about the nature of the product formed during the merger.”

The researchers will continue to monitor GW170817 to look for clues that point to either theory—or an entirely different trail. As Hajela looks at it: “This is an opportunity for us to study and see new physical processes happening.”

Correction (March 4, 2022): The headline was changed was from “galactic” to “extragalactic” because the kilanova was located outside of the Milky Way.