++ Gamma-ray burst that was heading for Earth detected - SouthernWolf.net

Gamma-ray burst that was heading for Earth detected

Posted on Thursday, September 18, 2008 @ 16:40:26 EDT in SpaceTime
by Southern

Roger Highfield

Gamma rays

Artist's impression of gamma-ray burst GRB 070714B

Astronomers say that a dying star's gamma ray burst, among the most violent events in the universe, was aimed squarely at Earth.

Some have speculated that the devastating radiation sent out by these cataclysmic events can sterilise galaxies, wiping out life forms before they have evolved sufficiently, as an explanation for why we have not detected any evidence of intelligent extraterrestrial life.

The star explodes in all directions like a bomb (making a supernova), but at the same time a narrow beam of gamma-rays and X-rays shoots out like a lighthouse beam or a search light.

Unparalleled data from satellites and observatories around the globe now show that the jet from one such powerful stellar explosion on 19th March was aimed almost directly at Earth. The event became bright enough for human eyes to see as a jet of material shot directly toward Earth at 99.99995 per cent the speed of light.

Fortunately, the extraordinary event, known as GRB 080319B took place 7.5 billion light years away, half way across the universe, concludes a study published today in the journal Nature.

This is too far away to have a sterilising effect. "It would probably have to be inside our own galaxy, or within about 30,000 light years, to be hazardous to life," says Prof David Burrows of Penn State University.

"A powerful gamma ray burst pointing at us could spell trouble even if it were a few thousand light years away - in other words, roughly 10 per cent across the galaxy," adds Prof Nial Tanvir of the University of Leicester.

"At this distance you might just be talking about an effect on the Earth's climate, like a nuclear winter scenario. Closer ones could lead to mass extinctions or even complete sterilisation."

Nasa's Swift satellite detected the explosion at 7:13 am UK time that morning and pinpointed its position in the constellation Boötes.

"This is yet another exciting result from the wonderfully productive Swift satellite," comments Dr Julian Osborne of Leicester University. "This burst was the optically brightest explosion ever seen."

In the journal Nature, Judith Racusin, a graduate student at Penn State and the lead author of the paper, and a team of 92 coauthors, report observations that began 30 minutes before the explosion and followed its afterglow for months, providing the most detailed portrait of a gamma-ray burst ever..

"At first, I thought something was wrong," Racusin recalls, because both Swift's X-Ray Telescope and its UltraViolet/Optical Telescope indicated they were effectively blinded by the brightness.

"Within minutes, as reports from other observers arrived, it was clear this explosion was an especially extraordinary event."

At the same moment Swift saw the burst, a Russian instrument on Nasa's Wind satellite also sensed the gamma rays. Simultaneously, a robotic wide-field optical camera called "Pi of the Sky" in Chile captured the burst's first visible light.

Within the next 15 seconds, the burst brightened enough to be visible in a dark sky to human eyes.

Dr Patricia Schady of the Mullard Space Science Laboratory, also part of the Swift team, says: "The GRB was created when a massive star ran out of nuclear fuel.

The star's core collapsed to form a black hole, driving powerful jets outward. These jets are amongst the fastest bulk flow of matter in the cosmos, moving close to the speed of light."

As the jets shoot into space, they strike gas previously shed by the star and heat it, which generates bright afterglows.

The team believes the jet directed toward Earth contained an ultra-fast component just 0.4 of a degree across, so the incredible energy of the gamma ray beam would spread out before it reached us.

"In spite of the enormous distance to this event, the optical and gamma ray light was still incredibly bright," says Prof Burrows.

Some have suggested that one such burst of gamma-rays may have caused one of Earth's worst mass extinctions, 443 million years ago.

Adrian Melott of the University of Kansas in Lawrence speculated that their energy would rip apart nitrogen and oxygen molecules, creating nitrogen oxides, notably the toxic brown gas nitrogen dioxide that colours smog.

A burst would produce enough of the gas to darken the sky, blotting out half the visible sunlight reaching the Earth. Nitrogen dioxide would also destroy the ozone layer, exposing surface life to a dangerous overdose of ultraviolet radiation from the sun for a year or more until the ozone recovered.

Even though bursts are detected almost every day, scattered randomly throughout the universe, there are no stars within 200 light years of our Solar System that are of the type destined to explode as a gamma ray burst.

"We think that a GRB is unlikely in our galaxy, because it appears that GRBs form from stars with low amounts of "metals" (elements heavier than helium), and young stars in our galaxy probably have too many metals to form a GRB," says Prof Burrows.

"So we think that this is one catastrophe that we are safe from."


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