A violent burst of X-rays from beyond the neighbourhood of the Milky Way temporarily blinded the sensors on the orbiting Swift satellite last month, reveal mission scientists.

Blasts of radiation like this result from the collapse of a massive star into a black hole; the energy is emitted as a gamma-ray burst (GRB), and are the most powerful form of explosion in the Universe. This particular GRB, which overwhelmed Swift on 21 June and thus earns it the catalogue number of 100621A, had been travelling for five billion years. Its light therefore began its journey before our Solar System took shape.

"This gamma-ray burst is by far the brightest light source ever seen in X-ray wavelengths at cosmological distances," says David Burrows, lead scientist for Swift's X-ray Telescope, which is a joint NASA, UK and Italian mission.

The intensity of the X-rays was unexpected – peaking at 143,000 X-ray photons per second, but only for 0.2 seconds. All the same, this was a staggering 140 times brighter than the brightest continuous X-ray source in the sky. "Just when we were beginning to think that we had seen everything that gamma-ray bursts could throw at us, this burst came along to challenge our assumptions about how powerful their X-ray emissions can be," says Swift's principal investigator Neil Gehrels.

Phil Evans from the University of Leicester comments that the burst was so bright that even the spacecraft didn't believe that this was a real event, and shut down some of its data-analysis software. "So many photons were bombarding the detector each second that it just couldn't count them quickly enough, it was like using a rain gauge and a bucket to measure the flow rate of a tsunami," he says.

In order to estimate the overall brightness scientists used a standard correction technique which samples photons at some distance from the overexposed centre. Evans, who wrote the analysis software in question, was the first to see the processed data. "When I first saw the strange data from this burst, I knew that I had discovered something extraordinary," he says. "It was an indescribable feeling when I realized, at that moment, that I was the only person who knew that this extraordinary event had occurred. Now, after our analysis of the data, we know that this burst is one for the record books."

The next brightest GRB is 2008's naked eye 080319B, which was seven times fainter but twice as far away as June 2010's event. Evans comments that even if you moved GRB 100621A to the same distance, it would still be one and a half times brighter than GRB 080319B.

After the initial burst GRBs slowly fade away, leaving an imprint at longer wavelengths. But despite 100621A's record breaking brightness in X-rays, it was nothing out of the ordinary in optical and ultraviolet wavelengths.