Extreme Gamma-Ray Burst

The first gamma-ray burst to be seen in high-resolution from NASA's Fermi Gamma-ray Space Telescope is one for the record books. The blast had the greatest total energy, the fastest motions and the highest-energy initial emissions ever seen.

"We were waiting for this one," said Peter Michelson, the principal investigator on Fermi's Large Area Telescope (LAT) at Stanford University. "Burst emissions at these energies are still poorly understood, and Fermi is giving us the tools to understand them."

This explosion, designated GRB 080916C, occurred at 7:13 p.m. EDT on Sept. 15, 2008, in the constellation Carina. This movie compresses about 8 minutes of Fermi LAT observations of GRB 080916C into 6 seconds. Colored dots represent gamma rays of different energies:

Above: A Fermi LAT movie of the extreme gamma-ray burst. The blue dots represent lower-energy gamma rays (less than 100 million eV); green, moderate energies (100 million to 1 billion eV); and red, the highest energies (more than 1 billion eV).

Fermi's other instrument, the Gamma-ray Burst Monitor, simultaneously recorded the event. Together, the two instruments provide a view of the blast's initial, or prompt, gamma-ray emission from energies between 3,000 to more than 5 billion times that of visible light.

Gamma-ray bursts are the universe's most luminous explosions. Astronomers believe most occur when exotic massive stars run out of nuclear fuel. As a star's core collapses into a black hole, jets of material -- powered by processes not yet fully understood -- blast outward at nearly the speed of light. The jets bore all the way through the collapsing star and continue into space, where they interact with gas previously shed by the star and generate bright afterglows that fade with time.

The first thing astronomers usually do after a gamma-ray burst is scramble to detect the fading afterglow. An afterglow's spectrum (i.e., its colors) can reveal the distance to the blast site. This is crucial information astronomers must have to calculate a gamma-ray burst's power.

Nearly 32 hours after the blast, a group led by Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, found the afterglow of GRB 080916C. Working quickly, before it could fade away, they measured the afterglow's spectrum using the Gamma-Ray Burst Optical/Near-Infrared Detector, or GROND, on the 2.2-meter telescope at the European Southern Observatory in La Silla, Chile.

According to their data, the explosion took place 12.2 billion light-years away.

"Already, this was an exciting burst," said Julie McEnery, a Fermi deputy project scientist at NASA's Goddard Space Flight Center in Greenbelt, Md. "But with the GROND team's distance, it went from exciting to extraordinary."

With the distance in hand, Fermi team members calculated that the blast exceeded the power of approximately 9,000 ordinary supernovae, if the energy was emitted equally in all directions. This is a standard way for astronomers to compare events even though gamma-ray bursts emit most of their energy in tight jets.

Coupled with the Fermi measurements, the distance also helps astronomers determine the speed of the gamma-ray emitting material. Within the jet of this burst, gas bullets must have moved at least 99.9999 percent the speed of light. This burst's tremendous power and speed make it the most extreme recorded to date.The team's results appear in the Feb. 19th online edition of the journal Science.