Ů��С��Ƶ

In less time than it takes you to get ready for work, some X-ray emitting space objects become hundreds of times brighter than usual, then dim back down. But what those objects are, astronomers haven’t yet figured out.

In 2003 and 2007, scientists detected two near a galaxy called NGC 4697. No one had found anything like them before or since – until astronomer at the University of Alabama in Tuscaloosa announced today that his team has caught two more such flares.

Irwin’s team originally planned to search through data from the to find star-sized black holes in groups of old stars around other galaxies. These black holes can up their X-ray emissions by a factor of five to 10 in around an hour as material swirls around the point of no return and gives off energy.

“But we found these extreme objects that varied by factors of 100 to 200,” says Irwin. “So it was a bit of an accident that we found the flares, as we were not originally looking for something so spectacular.”

In earlier X-ray observations of 70 galaxies, they found two such sources, around galaxies NGC 4636 and NGC 5128. The mystery objects both flared up in seconds and dimmed back to baseline X-ray brightness in about an hour. One erupted five times while telescopes happened to be watching, which means it probably acts up about every 1.8 days.

Excitable emitters

These new observations are very similar to the original flare spotted in 2003 and 2007, meaning their sources are probably similar objects.

“There’s a lot of consistency there, and that’s well-documented,” says at the Harvard-Smithsonian Center for Astrophysics. “That opens up the mystery for everyone, which is, ‘What are they?’ ”

Irwin and colleagues present a few ideas. Maybe they’re intermediate-mass black holes – which are rare and not well understood – devouring a meal. Or they could be neutron stars rapidly stealing material from a nearby companion.

“I think that people over the next weeks and months will come up with even more suggestions,” DiStefano says, and then do the necessary work to distinguish between the possibilities.

Whatever these excitable emitters are, the Milky Way doesn’t have anything like them, so we will just have to get to know the neighbours.