‘In 2005, a very strange event was observed. An unknown object, not detectable through visible light, released an intense flare of X-rays. It took about a minute for the flare to reach its full brightness, about 90 times brighter than its resting output and about a million times as bright as the Sun. The flare lasted for about an hour before petering out. Four years later, it flared up again.
X-ray flares are not unheard of, but this event defied classification. Astronomers normally look at the length of the flares as well as how often they occur to determine what kinds of processes produce them. These flares don’t match any known mechanism, making them mysterious indeed.
To find out more, a team of researchers decided to look over archival data from the Chandra and XMM-Newton space observatories. They wondered if similar phenomena are taking place anywhere else in the Universe. If so, it might provide clues about the nature of these strange flares. And the researchers weren’t disappointed. Their search, which included 70 nearby galaxies, turned up two more such flares…’
Source: Ars Technica