Article Posted on : - Feb 23, 2009

GRB 080916C's X-ray afterglow appears orange and yellow in this view that merges images from Swift's UltraViolet/Optical and X-ray telescopes. NASA image

WASHINGTON (BNS): Scientists with the help of NASA’s Fermi Gamma-ray Space Telescope witnessed the first gamma-ray burst in September last. The blast had the greatest total energy, the fastest motions and the highest-energy initial emissions ever seen, the scientists said in the online edition of the journal Science, which was published online on Thursday.

Peter Michelson, principal investigator on Fermi’s Large Area Telescope at Stanford University said that they were waiting for this to happen. “Burst emissions at these energies are still poorly understood, and Fermi is giving us the tools to understand them,” he said.

“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 generates bright afterglows that fade with time,” the astrophysicists said.

The spectacular blast, which occurred on September 15 last year in the Carina constellation, produced energies ranging from 3,000 to more than five billion times that of visible light, they said.

NASA said that nearly 32 hours after the blast, Jochen Greiner of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, led a group that searched for the explosion's fading afterglow. “The team simultaneously captured the field in seven wavelengths 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. In certain colours, the brightness of a distant object shows a characteristic drop-off caused by intervening gas clouds. The farther away the object is, the redder the wavelength where this fade-out occurs. This gives astronomers a quick estimate of the object's distance. The team’s follow-up observations established that the explosion took place 12.2 billion light-years away,” NASA said.

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

“With the distance in hand, Fermi team members showed 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,” the Goddard scientist said.

Coupled with the Fermi measurements, the distance also helps astronomers determine the slowest speeds possible for material emitting the prompt gamma rays. Within the jet of this burst, gas bullets must have moved at 99.9999 percent the speed of light. This burst's tremendous power and speed make it the most extreme recorded to date, the scientists said.

“One curious aspect of the burst is a five-second delay separating the highest-energy emissions from the lowest. Such a time lag has been seen clearly in only one earlier burst.

It may mean that the highest-energy emissions are coming from different parts of the jet or created through a different mechanism,” Michelson added.