In October 2014, in a spot the Chandra X-Ray Observatory had observed several times over 17 years, the blackness of space lit up with a sudden outburst of x-rays. In just a few hours' time, the source became brighter by a factor of 1,000. A day later it was gone. And astronomers aren't certain what caused it.
To put it in perspective, the x-ray burst captured by the space-based telescope was a few thousand times more luminous than the x-rays in the galaxy's whole history.
"I wish I knew what it was," said Ezequiel Treister, co-author of the paper published earlier this year in the Monthly Notices of the Royal Astronomical Society.
Finding new sources of x-rays like this in space isn't exactly rare. Things that come to an end in our universe tend to put on a big show. But this one didn't resemble anything astronomers had ever seen before.
The team of researchers considered the most likely possibilities: an intermediate black hole — about 1,000 times the mass of our own sun — ripped apart a white dwarf star; a neutron star, which is a small but dense star, merged with another neutron star or a black hole; or a massive star collapsed.
But none of those events fit the data exactly.
Adding to the confusion, the astronomers aren't exactly sure where the burst originated. They have a tentative location — a galaxy that is about 10.7 billion light-years away — but they're not certain that is the source.
So what is it?
In the neutron star scenario, a merge would produce gamma ray bursts (GRBs), jets that stream out into space. But this new x-ray eruption, though powerful, was about 100 times fainter than a typical GRB.
It could still be a GRB — one that we aren't looking at head-on, which would explain why it's fainter than most.
It could also be one with low luminosity, though Treister said that would be "very weird," since typically GRBs don't exhibit x-rays like this.
The other possibility is that the source of the GRB is farther away than the preliminary estimates.
As for the neutron star or white dwarf interacting with a black hole, Treister said that these are "extremely rare" and thus unlikely.
"It can also be an entirely new phenomenon," he said. "The problem is that we've only observed one."
The team went back through 17 years of Chandra data and found no similar occurrences. And to date, nothing like it has been found elsewhere.
The team had hoped it was a periodic event and would repeat, but they're still waiting. It could reappear at any time — a day from now, 10 days, a thousand or 10,000. Or never.
Treister said the next step is to look for more of these bursts, perhaps in similar galaxies. But since it may not be associated with the galaxy at all, it's a challenge to figure out where to look.
"Who knows? Maybe we are discovering an entirely new phenomenon that we didn't know was happening," Treister said. "Nature has this habit of surprising us. When we think we have pretty much everything figured out, it comes and shocks us with something new."