"Naked-eye" gamma ray burst was aimed squarely at Earth

Washington, September 11 : Data from satellites and observatories around the globe have shown that a jet from a powerful stellar explosion, which was witnessed on March 19 this year, was aimed almost directly at Earth.

NASA's Swift satellite detected the explosion - formally named GRB 080319B - on the morning of March 19, and pinpointed its position in the constellation Bootes.

The event, called a gamma-ray burst, became bright enough for human eyes to see. Observations of the event are giving astronomers the most detailed portrait of a burst ever recorded.

"Swift was designed to find unusual bursts," said Swift principal investigator Neil Gehrels at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "We really hit the jackpot with this one," he added.

A team of astronomers led by Judith Racusin of Penn State University, present their findings in a paper in the latest issue of the journal Nature, following work first presented at the May meeting of the Royal Astronomical Society.

The team concluded that the extraordinary brightness of the March 19 burst arose from a narrow jet that shot material directly toward Earth at 99.99995 percent of the speed of light.

The data clearly reveal the complexity of a GRB in which a narrow, ultra-fast jet is present within a wider, slightly slower jet.

At the same moment Swift saw the burst, the Russian KONUS instrument on NASA's Wind satellite also sensed the gamma rays and provided a wide view of their spectral structure.

A robotic wide-field optical camera called "Pi of the Sky" in Chile simultaneously 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. It briefly crested at a magnitude of 5.3 on the astronomical brightness scale.

Incredibly, the dying star was 7.5 billion light-years away.

Gamma-ray bursts are the universe's most luminous explosions. Most occur when massive stars run out of nuclear fuel.

As a star's core collapses, it creates a black hole or neutron star that, through processes not fully understood, drive powerful gas jets outward. These jets punch through the collapsing star.

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

The team believes that the jet directed toward Earth contained an ultra-fast component just 0.4 of a degree across. This core resided within a slightly less energetic jet about 20 times wider.

"It's this wide jet that Swift usually sees from other bursts," Racusin explained. "Maybe every gamma-ray burst contains a narrow jet, too, but astronomers miss them because we don't see them head-on," he added.

Such an alignment occurs by chance only about once a decade, so a GRB 080319B is a rare catch.

ANI