Hubble image
©NASA/ESA/N Tanvir/U of Leicester/A Fruchter/STScI
The afterglow of the most powerful explosion ever recorded is shown in this Hubble Space Telescope image. A few galaxies are also visible, but not the host galaxy for the explosion

The most powerful explosion ever observed may have been even more powerful than first thought. Nearly three weeks after the bright 'gamma-ray burst' occurred, it is still outshining its host galaxy, dumbfounding astronomers with its amazing longevity.

The cataclysmic event appears to have been caused by the collapse of an extremely massive star, perhaps as massive as 50 Suns, lying about 7.5 billion light years from Earth.

For a brief period on 19 March, it was easily the most distant object visible to the naked eye, lying thousands of times farther than the nearby Triangulum Galaxy, which normally holds that title. It was intrinsically 2.5 million times brighter than the next brightest explosion, a supernova that occurred in 2005.

Astronomers had hoped to image the star's host galaxy with the Hubble Space Telescope, but a Hubble image taken on 7 April revealed that the host galaxy was still hidden in the glare of the explosion that had occurred nearly three weeks earlier.

These bursts are thought to be observed when jets of matter moving at nearly the speed of light shoot out of the star in the direction of Earth. These send gamma rays our way, along with visible light produced where the jets heat up surrounding gas.

Tightly focused

Some astronomers have suggested that brighter gamma-ray bursts owe their brilliance to more tightly focused jets. These initially focus more of their energy towards Earth, but the effect rapidly wears off as the jets slow down.

As a result, brighter gamma-ray bursts are expected to fade more quickly, a trend that has been observed in previous bursts. But this gamma-ray burst is bucking the trend, casting doubt on whether this burst's unusual brilliance can be explained by especially tight jets.

On the other hand, if the burst did not have very tight jets, then its energy would have to be "really huge" to explain its brightness from such a distance, says Nial Tanvir of the University of Leicester in the UK, who is leading the Hubble observations.

"It's looking like a serious problem," he told New Scientist.

This could be a sign that - for unknown reasons - the explosion converted a much larger fraction of its energy into radiation than gamma-ray bursts normally do, Tanvir says, although he says it is too early to make firm conclusions.

Hubble will make another attempt to view the host galaxy in May.