Breaking News Emails

Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Update for 2:30 p.m. ET May 28: After the buzz broke, astronomers figured out that the flash was a false alarm. Check out the update.

Earlier: A powerful flash of gamma rays, apparently coming from the Andromeda Galaxy, has set astronomers abuzz over the possibility that a clashing pair of neutron stars or a bright X-ray source is acting up a mere 2.5 million light-years away.

The event was registered by NASA's Swift satellite on Tuesday, and within hours the report sparked a flurry of Twitter updates.

This mosaic of the Andromeda Galaxy, also known as M31, merges 330 individual images taken by the Ultraviolet/Optical Telescope aboard NASA's Swift spacecraft.Stefan Immler and Erin Grand / NASA

At first, astronomers speculated that the flash was a gamma-ray burst, which ranks among the most powerful explosions in the universe. Such blasts, thought to be caused by the collision of neutron stars or black holes, are extremely rare — and none has been detected so close before.

That kicked off rounds of black humor on Twitter:

But there's no danger of the Hulk getting angry, or any other apocalyptic outcome. If the event does turn out to be a gamma-ray burst, it would simply provide the best opportunity yet to study one of the most exotic events in the cosmos.

As time went on, some astronomers said the flash could have been caused by a somewhat less exotic type of ultraluminous X-ray source, also known as a ULX. Such sources have been seen before, even in the Andromeda Galaxy (which is also known as M31). They're thought to be huge black holes that are gobbling up matter from companion stars, and sending out X-ray jets as a result.

Another possibility is that the burst came from a low-mass X-ray binary star system, or LMXB.

Such outbursts are still violent and worthy of an astronomer's gasps — but they're not on a par with gamma-ray bursts. If Tuesday's event came from a ULX, the X-ray emissions would be more sustained.

Astronomers scrambled to their telescopes (and computer screens) to try to resolve the nature of the source — and the answers are likely to become clearer within the next day or two. To get in on the buzz, follow the Twitter hashtags #GRBm31 and #ULXm31.