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February
20, 2009: 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). Credit:
NASA/DOE/Fermi LAT Collaboration. [Quicktime
video]
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.
 Right:
The fading afterglow detected by GROND. [Larger
image]
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.
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Editor: Dr.
Tony Phillips | Credit: Science@NASA
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