It came suddenly from the distant reaches of the Constellation Sagittarius, some 50,000 light years away. For a brief instant, a couple of tenths of a second, on December 27, 2004 an invisible burst of energy the equivalent of half a million years of sunlight shone on Earth. Many orbiting satellites electronics were zapped and the Earth's upper atmosphere was amazingly ionized from a massive hit of gamma ray energy.
The source of the invisible attack was a rare magnetar SGR 1806-20 on the other side of the Milky Way. These soft gamma ray repeaters, SGRs, occur when twisted magnetic fields attempt to re-align themselves and crack the magetar's crust releasing the awesome burst or pulse of energy with a death-zone of a few light years. Magnetars have magnetic fields 1000 times those of ordinary pulsars -so powerful as to be lethal at a distance of 1000 kilometers.
Atronomers have catalogued well over 1000 pulsars, and estimate the number of quiet netron stars to be vastly more at some 100 million given the 10-billion-year life of the Milky Way's disk. The odds are that one is nearby, gliding sliently past Earth, of no danger. The tonest fraction of neutron stars have morphed into magnetars, be;ived to be the offspring of the most maasive stars, hypergiants that don't have enough mass to evolve into black holes.
Fortunately for Earth, the nearest GRB candidate seems to be thousands of light-years away. Maybe...
Data from satellites and observatories around the globe showed a jet from a powerful stellar explosion witnessed on March 19, 2008 aimed almost directly at Earth. NASA's Swift satellite detected the explosion - formally named GRB 080319B - at 2:13 a.m. EDT that morning and pinpointed its position in the constellation Bootes. The 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, Md. "We really hit the jackpot with this one."
In a paper that appeared in Nature, Judith Racusin of Penn State University and a team of 92 coauthors reported on observations across the spectrum that began 30 minutes before the explosion and followed its afterglow for months. The team concludes the burst's extraordinary brightness arose from a jet that shot material directly toward Earth at 99.99995 percent the speed of light.
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.
Telescopes around the world already were studying the afterglow of another burst when GRB 080319B exploded just 10 degrees away. TORTORA, a robotic wide-field optical camera operated in Chile with Russian-Italian collaboration, also caught the early light. TORTORA's rapid imaging provided the most detailed look yet at visible light associated with a burst's initial gamma-ray blast.
Immediately after the blast, Swift's UltraViolet and Optical Telescope and X-Ray Telescope indicated they were effectively blinded. Racusin initially thought something was wrong. Within minutes, however, as reports from other observers arrived, it was clear this was a special event.
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 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."
Such an alignment occurs by chance only about once a decade, so a GRB 080319B was a rare catch.
Source: NASA/Goddard Space Flight Center