May 6, 1998
A recently detected cosmic gamma ray burst released a hundred times more energy than previously theorized, making it the most powerful explosion since the creation of the universe in the Big Bang.
"For about one or two seconds, this burst was as luminous as all the rest of the entire universe," said Caltech professor George Djorgovski, one of the two principal investigators on the team from the California Institute of Technology, Pasadena, CA.
The team measured the distance to a faint galaxy from which the burst originated at about 12 billion light years from the Earth. The observed brightness of the burst despite this great distance implies an enormous energy release. The team's findings appear in the May 7 issue of the journal Nature.
The burst was detected on Dec. 14, 1997, by the Italian/Dutch BeppoSAX satellite and NASA's Compton Gamma Ray Observatory satellite. The Compton observatory provided detailed measurements of the total brightness of the burst, designated GRB 971214, while BeppoSAX provided its precise location, enabling follow-up observations with ground-based telescopes and NASA's Hubble Space Telescope.
"The energy released by this burst in its first few seconds staggers the imagination," said Caltech professor Shrinivas Kulkarni, the other principal investigator on the team.
The burst appears to have released several hundred times more energy than an exploding star, called a supernova, until now the most energetic known phenomenon in the universe. Finding such a large energy release over such a brief period of time is unprecedented in astronomy, except for the Big Bang itself.
"In a region about a hundred miles across, the burst created conditions like those in the early universe, about one millisecond (1/1,000 of a second) after the Big Bang," said Djorgovski.
This large amount of energy was a surprise to astronomers. "Most of the theoretical models proposed to explain these bursts cannot explain this much energy," said Kulkarni. "However, there are recent models, involving rotating black holes, which can work. On the other hand, this is such an extreme phenomenon that it is possible we are dealing with something completely unanticipated and even more exotic."
Gamma-ray bursts are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They were first discovered by U.S. Air Force Vela satellites in the 1960s. Since then, numerous theories of their origin have been proposed, but the causes of gamma-ray bursts remain unknown. The Compton observatory has detected several thousand bursts so far.
The principal limitation in understanding the bursts was the difficulty in pinpointing their direction on the sky. Unlike visible light, gamma rays are exceedingly difficult to observe with a telescope, and the bursts' short duration exacerbates the problem. With BeppoSAX, scientists now have a tool to localize the bursts on the celestial sphere with sufficient precision to permit follow-up observations with the world's most powerful ground-based telescopes.
This breakthrough led to the discovery of long-lived "afterglows" of bursts in X-rays, visible and infrared light, and radio waves. While gamma-ray bursts last only a few seconds, their afterglows can be studied for several months. Study of the afterglows indicated that the bursts do not originate within our own galaxy, the Milky Way, but rather are associated with extremely distant galaxies.
Both BeppoSAX and NASA's Rossi X-ray Timing Explorer spacecraft detected an X-ray afterglow. BeppoSAX precision led to the detection of a visible light afterglow, found by a team from Columbia University, New York, NY, and Dartmouth College, Hanover, NH, including Professors Jules Halpern, David Helfand, John Torstensen, and their collaborators, using a 2.4-meter telescope at Kitt Peak, AZ, but no distance could be measured from these observations.
As the visible light from the burst afterglow faded, the Caltech team detected an extremely faint galaxy at its location, using one of the world's largest telescopes, the 10-meter Keck II telescope at Mauna Kea, Hawaii. The galaxy is about as faint as an ordinary 100 watt light bulb would be as seen from a distance of a million miles.
Subsequent images taken with the Hubble Space Telescope confirmed the association of the burst afterglow with this faint galaxy and provided a more detailed image of the host galaxy.
The Caltech team succeeded in measuring the distance to this galaxy, using the light-gathering power of the Keck II telescope. The galaxy is at a redshift of z=3.4, or about 12 billion light years distant (assuming the universe to be about 14 billion years old).
From the distance and the observed brightness of the burst, astronomers derived the amount of energy released in the flash. Although the burst lasted approximately 50 seconds, the energy released was hundreds of times larger than the energy given out in supernova explosions, and it is about equal to the amount of energy radiated by our entire Galaxy over a period of a couple of centuries. Scientists say it is possible that other forms of radiation from the burst, such as neutrinos or gravity waves, which are extremely difficult to detect, carried a hundred times more energy than that.
NASA is planning two missions to further investigate gamma- ray bursts: the High Energy Transient Experiment II (HETE II), scheduled to launch in the fall of 1999, and the Gamma Ray Large Area Space Telescope (GLAST), scheduled to launch in 2005. HETE II will be able to precisely locate gamma-ray bursts in near real- time and quickly transmit their locations to ground-based observatories, permitting rapid follow-up studies. GLAST will detect only those gamma-ray bursts that emit the highest energy gamma rays, and will be able to locate them with sufficient precision to permit coordinated observations from the ground. Because not much is known about the bursts at these high energies, the observations may permit researchers to choose among competing theories for the origin of gamma-ray bursts.
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NOTE TO EDITORS:
Images of the GRB 971214 field are available at: FTP://PAO.GSFC.NASA.GOV/newsmedia/GRB/
Information on the BeppoSAX spacecraft is available at: http://www.sdc.asi.it/
Information on the Compton Gamma Ray Observatory is available at: http://cossc.gsfc.nasa.gov/cossc/descriptions/cgro.html
Information on Gamma Ray Bursts is available at: http://cossc.gsfc.nasa.gov/cossc/nasm/VU/overview/bursts/bursts.html
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