The burst of X-rays was so strong that it temporarily blinded a NASA satellite.
A huge burst of X-rays detected by NASA's Swift Telescope was the most powerful emission ever seen.
The blast may have stemmed from a massive star's collapse into a black hole.
Scientists still don't know why this blast was more than 10 times brighter than others previously detected.
A burst of X-rays that temporarily blinded a NASA satellite last month was the most powerful blast ever recorded, leaving astronomers mystified about its origin.
The Swift X-ray observatory, which tracks gamma-ray bursts, was so overwhelmed by the June 21 event that computer analysis of data collected during the burst discarded the information as junk.
It wasn't until a research assistant, who was returning from vacation, went back to look at the data dropout that scientists had an inkling of what happened.
"We actually didn't realize at first how bright the X-ray emission was," Swift lead researcher Neil Gehrels at NASA's Goddard Space Flight Center in Maryland told Discovery News.
"If someone had asked me before Swift launched how bright these X-ray afterglows of gamma ray bursts would be, I'd never have guessed it'd be this bright," Gehrels added. "It really knocked our socks off."
The burst was about 200 times brighter than astronomers' benchmark for X-ray radiation, the supernova remnant known as the Crab Nebula, and it stemmed from a galaxy about 5 million light-years away.
After spending a few weeks verifying Swift's health and re-checking the data, scientists have moved on to a more taxing problem: figuring out what triggered the blast.
Gamma-ray bursts that sire these types of X-ray blasts typically occur when a massive star collapses into a black hole.
"We're trying to grapple with what happened here. The best thinking is that it was still a burst of a black hole. But when this particular black hole was born, there was something particularly violent going on in the outflow," Gehrels said.
Scientists believe gamma-ray bursts occur about one or twice a day somewhere in the universe. Swift, which continually scans the sky for busts, typically picks up one or two a week. When it detects a burst, the observatory automatically repositions its X-ray telescope to home in on the target.
The June 21 blast, which lasted about a minute, was nearly over when the X-ray telescope got in position. The photons were coming in so fast and so powerfully that they oversaturated the sensors.
"We got the telescope there with about 10 seconds to spare. If we had been a little later, we wouldn't have seen it," Penn State astronomer David Burrows, lead scientist for Swift's X-ray telescope, told Discovery News.
At its peak, the gamma-ray burst produced 145,000 X-ray photons a second -- about 10 to 15 times brighter than anything previously seen by Swift.