Mysterious cosmic explosion detected by astronomers

In October 2014, in a spot the Chandra X-Ray Observatoryhad 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 telescopewasa few thousand timesmore luminous than the x-rays in the galaxy's whole history.

I wish I knew what it was.- EzequielTreister

"I wish I knew what it was," saidEzequielTreister, co-author of the paper published earlier this yearin 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; ora massive star collapsed.

But none of those events fit the data exactly.

Possible explanations

So what is it?

Inthe neutron star scenario, amerge wouldproduce gamma ray bursts (GRBs),jets that streamout into space.But this newx-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, thoughTreistersaid that would be "very weird," since typicallyGRBsdon't exhibit x-rays like this.

Nature has this habit of surprising us.- EzequielTreister

The other possibility is that the source of the GRB isfarther away than the preliminaryestimates.

Asfor the neutron star or white dwarf interacting with a black hole,Treistersaid that these are "extremely rare" and thus unlikely.

"It can also be an entirely new phenomenon,"hesaid. "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.

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The teamhad 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 thousandor 10,000. Or never.

Treistersaid 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 wedidn't know washappening," 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."