NASA / Sunday January 31, 1999
First ever live gamma ray burst observed
Astronomers racing the clock managed to take the first-ever optical
images of one of the most powerful explosions in the Universe -- a
gamma ray burst -- as it was occurring on Saturday, Jan. 23, 1999.
Gamma ray bursts produce more energy in a very short period than the
rest of the entire Universe combined.
Because such bursts occur with no warning and typically last for just
a few seconds, quick detection by orbiting spacecraft and instant
notification to astronomers are critical in order to catch the bursts
in the act.
The gamma-ray-burst detectors of the Burst and Transient Source
Experiment (BATSE) onboard NASA's orbiting Compton Gamma Ray
Observatory detected the beginning of a bright gamma ray burst. As
the burst was still in progress, computers determined a rough
location and radioed the position to the Gamma Ray Burst Coordinates
Network (GCN), based at NASA's Goddard Space Flight Center,
Greenbelt, MD. The position was immediately forwarded via the GCN to
astronomers at ground based observatories throughout the world.
Just 22 seconds later the Robotic Optical Transient Search Experiment
(ROTSE) in Los Alamos, NM, operated by a team led by Dr. Carl Akerlof
of the University of Michigan, was in position and took images of the
patch of sky where the burst was reported. Their equipment is
assembled from 35 mm camera lenses and parts culled from the amateur
astronomy market. The first picture showed a brightening new star
within the sky region where the burst was reported.
Five seconds later, the burst achieved peak brightness, reaching 9th
magnitude, about 16 times fainter than the human eye can see, but
easily visible in an amateur telescope. Within eight minutes of the
initial detection, the burst had faded by a factor of 100 below its
maximum brightness. "I was amazed," Akerlov said. "At best, we
expected something really dim optically, at the limit of our
sensitivity. Instead we found a whopper."
"If this burst had originated in the Milky Way Galaxy, it would have
lit up the night sky," said Dr. Alan Bunner, Director of NASA's
Structure and Evolution of the Universe science theme at NASA
Headquarters.
The event was also recorded by instruments aboard the Italian-Dutch
BeppoSAX satellite, which obtained a much more accurate position for
the burst within a few hours of its onset. It was this more precise
location information that the ROTSE team used to find the burst in
their images.
"This is the Holy Grail for the Gamma Ray Burst Coordinates Network,"
said Dr. Scott Barthelmy, the astronomer at Goddard, who developed
and runs the network. "Optical telescopes had seen the afterglow of a
burst, but never the burst itself. This observation will help us
understand the physical processes behind the bursting."
Within three hours of the gamma ray burst, a team of astronomers led
by Dr. Stephan Odewahn, and Profs. Shri Kulkarni and George
Djorgovski of the California Institute of Technology used the 60-inch
Mt. Palomar telescope to find a fading optical counterpart to this
gamma ray burst, helped by the precise localization provided by
BeppoSAX.
The next night, a joint team led by Dr. D. Kelson of the Carnegie
Institution of Washington, using the Keck II 10-meter telescope
located at Mauna Kea, HI, found that the distance to the burst is
about nine billion light years, more than half way to the edge of the
observable Universe.
Astronomers are not certain what produces gamma ray bursts, but
possible causes include the mergers of two neutron stars, two black
holes, or a neutron star and a black hole, or the explosion of a
so-called hypernova. A hypernova is a theorized type of supernova or
exploding star.
"The optical emission was about 10,000 times brighter than ever
observed, something you could see with a pair of good binoculars,"
said Dr. Neil Gehrels, Project Scientist of the Compton Observatory.
"Theorists will have a field day trying to explain this phenomenon."
Dr. Gehrels said the simultaneous observation of the burst in optical
and gamma ray energies might open the door to a whole new generation
of instruments like ROTSE, which is a fully automated telescope that
can respond to information about transient celestial sources
instantly. Orbiting telescopes detect several hundred gamma ray
bursts each year.
The ROTSE project is designed and operated by a collaboration of
astrophysicists from the University of Michigan and the Department of
Energy's Los Alamos and Lawrence Livermore National Laboratories. The
Principal Investigator for BATSE is Dr. Gerald Fishman at NASA's
Marshall Space Flight Center, Huntsville, AL. The National Science
Foundation provided funding for observations at Keck II.