Record-Breaking X-ray Blast Briefly Blinds Space Observatory

Scientists from the University of Leicester have played a leading role in an international team that has identified a dazzling burst of X-rays – the brightest ever detected from an object outside our own galaxy.

The X-rays had been travelling for over 5 billion years before being detected by the X-ray telescope on board the Swift satellite: a joint NASA, UK, and Italian mission. The X-ray camera, provided by University of Leicester scientists, was temporarily dazzled so bright was the flash.

The event during the year in which Leicester marked 50 years of space science gave scientists extra cause for celebration.

The source of the X-rays was a Gamma-Ray Burst: a violent eruption of energy from a massive star collapsing into a black hole. "Gamma Ray Bursts are the most powerful explosions in the Universe," said Julian Osborne, the head of the Swift team at Leicester, "but their radiation is not limited to Gamma Rays, and the X-ray telescope on Swift has produced a wealth of ground-breaking scientific results."

And now a new record-holder as well, "This gamma-ray burst is by far the brightest light source ever seen in X-ray wavelengths at cosmological distances," said David Burrows, senior scientist and professor of astronomy and astrophysics at Penn State University and the lead scientist for Swift's X-ray Telescope.

Although Swift was designed specifically to study Gamma Ray Bursts, the instrument was not intended to handle an X-ray blast this bright. "The intensity of these X-rays was unexpected and unprecedented," said Neil Gehrels, Swift's principial investigator, based at NASA's Goddard Space Flight Center. "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."

"The burst was so bright that when it first erupted our data-analysis software went into shut-down mode, assuming that there must have been a glitch somewhere in the software," explained Phil Evans, the University of Leicester scientist who wrote the analysis software in question. "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."

At the time of the burst, Dr. Evans was on holiday in the Lake District, so only became aware of the problem when he got home. After getting back in to work he tweaked the software, which recovered the data and captured the evolution of the burst over time. This revealed that the blast's brightness peaked at 143,000 X-ray photons per second during its fleeting period of record-breaking brightness; 14 times brighter than the brightest continuous X-ray source in the sky.

As Gamma-Ray Bursts fade away they leave an afterglow at longer wavelengths, including the optical and ultraviolet. Surprisingly, although the energy from this burst was the brightest ever in X-rays, it was merely ordinary in optical and ultraviolet wavelengths.

The Swift scientists were able to estimate the overall brightness of GRB 100621A by sampling the photons at some distance from its overexposed centre -- a standard correction technique. Scientists who study the Sun use a similar approach to observe the Sun's corona by blocking out its much-brighter centre. For this burst, the correction that had to be applied was more than 150 times that applied to a typical burst.

Automated analysis of the Swift XRT data is routinely performed at the University of Leicester in the United Kingdom, and used by scientists across the globe. Phil Evans, after tweaking his code, was the first to see the processed data from the beginning of this burst. "When I first saw the strange data from this burst, I knew that I had discovered something extraordinary," he said. "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." Evans tweets about his work on the Swift team as @Swift_Phil, and his followers were among the first to hear of this event.

Other members of the research team include Tilan Ukwatta at NASA Goddard Space Flight Center and Valerio D'Elia and Giulia Stratta at the ASI Science Data Center in Italy.

SWIFT WEB SITE: http://www.swift.ac.uk