GAMMA RAY BURST IMAGED FOR FIRST TIME
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.
Submitted by Brian Fraser