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Saturday, March 22, 2008

From Halfway to the Beginning of the Universe


Gamma-Ray Burst Visible to Naked Eye
How far can you see with your own eyes on a clear night? Would you believe seven billion light years?

Scientists have detected an interstellar explosion so bright that it was briefly visible to the naked eye—from 7.5 billion light-years away.

Viewers looking at the right patch of night sky on Wednesday would have seen several afterglows from the massive gamma-ray burst, slightly brighter than the faintest visible stars.

NASA's Swift satellite captured the unprecedented spectacle using its X-Ray Telescope (left) and Optical/Ultraviolet Telescope (right). The burst was named GRB 080319B, because it was the second of four bursts detected that day—a first for Swift.

GRB 080319B, located more than halfway across the visible universe, crushes the previous record holder for most distant object visible without assistance by three orders of magnitude. That would be the galaxy M33, located just 2.9 million light-years from Earth.

"This burst was a whopper," Swift principal investigator Neil Gehrels, of NASA Goddard Space Flight Center in Greenbelt, Maryland, said in a statement. "It blows away every gamma-ray burst we've seen so far."

Gamma-ray bursts occur when massive stars run out of fuel and collapse, rapidly pouring out enormous amounts of high-energy radiation and particles. Acting like high-power cosmic blowtorches, the particle jets can also heat interstellar clouds to create bright afterglows, according to scientists.

The bursts are the most brilliant occurrences in the universe after the big bang. GRB 080319B's afterglows, for instance, shone 2.5 million times more intensely than the brightest supernova on record.

Researchers are still unsure why GRB 080319B was so bright. Some theories include an originating star with an unusual mass, spin, or magnetic field or an especially concentrated energy jet.
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Early Wednesday morning, a spot of light just barely visible to the human eye (about fifth magnitude in astronomical parlance) appeared in the constellation Bo├Âtes. Astronomers say it was the toasted remains of one of the most titanic examples yet of the explosions known as gamma-ray bursts. News about the burst, in a galaxy seven billion light years away, began circulating by e-mail in the astronomical community when it was detected by NASA’s Swift satellite on March 19.

Gamma ray bursts are some of the most violent and enigmatic events in nature. Astronomers surmise that they might mark the implosion of a massive star into a black hole, or the collision of a pair of dense neutron stars.

The visible glow from this burst, said Neil Gehrels of NASA’s Goddard Space Flight Center, was 10 million times as bright as a supernova at that same distance. The universe is some 14 billion years old, which means that the news of this cataclysm has been on its way to us for half the age of the universe. Whatever stars went to their grave then have been dead since before the Sun and Earth were born.

The burst, which has now been dubbed the “naked-eye burst” by astronomers, was one of four that day to be detected by Swift, which has been patrolling the heavens since 2004 for the invisible gamma rays streaming from these blasts and relaying information and precise coordinates to a worldwide network of observers and telescopes. Dr. Gehrels said it was the most intense burst that Swift had yet seen.

Alerted by Swift, a myriad of telescopes on the ground swung into action, some of them operating completely robotically, which as Dr. Gehrels noted, is convenient at an early morning hour. Among those recording and inspecting the burst was one of the giant eight-meter-diameter telescopes of the Very Large Telescope at the European Southern Observatory on Cerro Paranal, in Chile. Spectral measurements of the glow’s redshift (the spectral shift due to motion away from us in the expanding universe) allowed the astronomers to estimate its surprisingly large distance.

That seven billion light years, astronomers say, would have been far and away the record for long-distance sight by the naked eye, at least in the present sky — had anybody seen it. So far, according to Dr. Gehrels, there is no report that anybody did. Within an hour, the glow had faded below the range of human visibility.

“It was an amazing burst, and we are having a lot of fun with it,” said Dr. Gehrels, who said that he and a large group of collaborators are preparing a quick report to submit to Nature.

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