Astronomers have found a strange flash of X-rays in a distant galaxy, and they are struggling to work out what caused it.

The event was first observed by NASA’s Chandra X-ray Observatory in October 2014. Its location in a galaxy 10.7 billion light-years away was later deduced by the Hubble and Spitzer Space Telescopes.

What the astronomers saw was a point in the sky known as the Chandra Deep Field-South (CDF-S) that became 1,000 times brighter in a few hours. After a day, it faded almost entirely out of view. A study describing the findings is to be published in the Monthly Notices of the Royal Astronomical Society.

“Ever since discovering this source, we’ve been struggling to understand its origin,” said lead author Franz Bauer of the Pontifical Catholic University of Chile in Santiago, Chile in a statement. “It’s like we have a jigsaw puzzle but we don’t have all of the pieces.”

No such event has ever been seen by the Chandra observatory before or after. And in two and a half months of observing this source, spread out over 17 years, it never brightened again.

While at a loss to explain it, there are some possibilities. Two of these would involve it being a gamma-ray burst (GRB) event, huge explosions that can be triggered by the collapse of a massive star. GRBs can also be caused by stars merging with other stars, or with a black hole. These can produce powerful jets that, if pointed towards us, appear for a brief moment of time.

For this particular source, though, one possibility is that it may have been a GRB that was not pointed directly towards us. Another is that it may be a GRB in a more distant galaxy. And a third idea is that it’s the result of a medium-sized black hole eating a white dwarf star.

“However, none of the above scenarios can completely explain all observed properties,” the researchers write in their paper. Thus, they suggest this might be a “new type of variable phenomena whose nature remains to be determined.”

To get to the bottom of it, astronomers will be hoping to find similar events with other telescopes like ESA’s XMM-Newton or NASA’s Swift satellite. But for now it adds to a number of things, like fast radio bursts (FRBs), that we just aren’t sure about in the universe.