Unparalleled data from satellites and observatories around the globe show that the jet from a powerful stellar explosion this past spring was aimed almost directly at Earth. The event, called a gamma-ray burst (GRB), became bright enough for human eyes to see. The observations give astronomers the most detailed portrait of a GRB ever made.
In a paper to appear in Thursday's issue of the journal "Nature," UNLV Associate Professor of Astronomy Bing Zhang and a team of 92 coauthors report observations that began 30 minutes before the explosion and followed its afterglow for months. They conclude that the burst's extraordinary brightness arose from a jet that shot material directly toward Earth at 99.99995 percent the speed of light.
Zhang worked with his colleagues at Penn State University (Judith Racusin, Xuefeng Wu, Peter Meszaros, and David Burrows) to propose a theoretical model to interpret this unusual event. According to their theory, the GRB has two jet components beaming towards the earth. The core component was only 0.4 degree across, but was very bright and moved ultra-fast. It resided within a slightly less energetic jet about 20 times wider.
"The broad component is more typical of other bursts", says Zhang. "Perhaps every gamma-ray burst has a narrow jet, but astronomers miss it most of the time. The nearly head-on alignment with earth is what made it visible - something that occurs by chance only about once a decade."
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.
NASA's Swift satellite detected the explosion - formally known as GRB 080319B - on March 19 at 2:19 a.m. EDT and pinpointed the blast's position in the constellation Bo?tes. "Swift was designed to find unusual bursts," says Swift principal investigator Neil Gehrels at NASA's Goddard Space Flight Center in Greenbelt, Md. "We really hit the jackpot with this event."
These jets represent the fastest bulk flow of matter in the cosmos. They rip through a star at nearly 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.
At the same moment that the Swift satellite saw the burst, the Russian KONUS detector on NASA's Wind satellite also sensed the gamma rays and provided a wide view of their spectral structure. Within the next 15 seconds, the blast became visible in a dark sky to human eyes. It briefly crested at magnitude 5.3 on the astronomical brightness scale. Two robotic wide-field optical cameras in Chile observed the brief flash: "Pi of the Sky," which is operated by the Center for Theoretical Physics in Warsaw, Poland, and TORTORA, which is run by a Russian-Italian collaboration.
Swift is managed by NASA's Goddard Space Flight Center. It was built and is being operated in collaboration with Penn State, the Los Alamos National Laboratory, and General Dynamics in the U.S.; the University of Leicester and Mullard Space Sciences Laboratory in the United Kingdom; Brera Observatory and the Italian Space Agency in Italy; plus partners in Germany and Japan.
