Two thousand arrived a little early for astrophysicists at NASA's Marshall Space Flight Center. Not the year, but the 2000th gamma-ray burst recorded by the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory.
Since 1991, BATSE has recorded a burst of gamma radiation about every day or so, on average. This fall, the BATSE team started counting down as the total climbed into the 1970s. The 2000th burst was expected around the end of the year, but the rate picked up a little. Although a complete record has not been compiled, another 1,000 bursts have been recorded by instruments other than BATSE.
BATSE Burst 2000 arrived Friday, Dec. 12, not with a bang, but with a whimper.
"It wasn't as big as most of the ones we see," said Dr. Gerald Fishman, the BATSE principal investigator. "It was smaller than average. Still, it adds to our volume of knowledge that eventually will let us decipher what these things are."
Gamma rays are electromagnetic radiation, the same as radio waves and visible light, but with much greater energy. A gamma ray can carry a million times the energy of a ray of visible light.
Thus, a burst of gamma rays indicates that something exceptionally violent has occurred. Astrophysicists now believe that each burst releases as much energy in 10 seconds as our sun will release in 10 billion years. Fortunately, bursts also seem to be quite distant and - as a result - far in the past of the universe.
There's nothing magic about number 2000, but it is a nice reminder that these are common events. Since mankind started recording events in the sky, more than 2 million gamma-ray bursts have gone unnoticed (assuming that the average rate holds and nature did not turn them on in time for humankind's Space Age).
The variety of events seen by BATSE includes:
- Burst 2000, which made its brief appearance and faded.
- One of the longest, smoothest bursts on Dec. 8, 1997 (GRB 971208). It took nearly two minutes to reach its peak and 15 minutes to trail off to background levels. Most bursts peak within a few seconds and trail off in tens of seconds. The brightness here was about the average of most bursts, though.
- A cluster of four bursts that appeared to come from the same region of the sky, suggesting some sort of connection, was recorded Oct. 26 and 27, 1996. The burst pattern has been a non-pattern, so even getting two in the same region and time would be a rare coincidence. Four are a real puzzler.
- The brightest burst, GRB 960924, which lit up the sky several hundred times brighter than the apparent brightness of the Crab Nebula (a standard candle in astrophysics) on Sept. 24, 1996. Many bursts, like the latest, are rated at just a few Crabs.
- Not everything that glitters in the gamma-ray spectrum is a true burst. BATSE has helped scientists discover a new class of objects called bursting pulsars, and track down and discover >soft gamma-ray repeaters. And it discovered that terrestrial thunderstorms often emit gamma rays in addition to light and radio noise.
Right now, the BATSE team and other astrophysicists are hot on the trail of No. 2002 (properly known as GRB 971214) which appeared Sunday. The burst was also detected and located by the Beppo SAX and Roentgen X-ray Timing Explorer satellites. Scientists now are hunting it with optical telescopes.
In the astrophysics community, gamma-ray bursts have risen in status from minor oddity to major mystery. Like the movie star who becomes an "overnight success," years of work preceded fame.
Gamma-ray bursts were discovered in data from a satellite designed to watch for clandestine nuclear weapons tests in space. Initially the data were classified. About the time that scientists determined that bursts were coming from beyond the solar system, the Department of Defense declassified the discovery and the astrophysics community at large started puzzling over it. By 1973, just 16 bursts had been recorded.
Because bursts rose and disappeared quickly, and never had an apparent visible object to go with them, no one knew exactly where to point a telescope to catch one in the act. So, bursts remained a minor mystery that few people knew.
One tabloid newspaper, though, published a lurid story in 1977 about how Earth's atmosphere barely saved it from a burst of gamma radiation that came scorching across space. In fact, the flash - by the time it got here - didn't have enough energy as a decent dental X-ray.
The headlines have been more respectable in recent years, with bursts being cited in the January 1998 Discover magazine as one of the Top 100 science stories of 1997.
The event was observed by spacecraft across the solar system. The difference in arrival times showed that the origin definitely was outside the solar system. In time, it heightened curiosity about bursts and helped NASA decide to select BATSE as one of four instruments for the Compton Observatory.
Since its launch in 1991, BATSE has done basically what scientists wanted of it. It has shown where they originate. The answer was unexpected since the BATSE team though they came from within the Milky Way. When the data showed that the bursts were evenly scattered across the sky, the team decided that the origin had to be throughout the universe. Other clues inside the burst signatures led them to believe that bursts were so distant that they might be near the edge of creation.
And the mystery had become deep enough that in September 1997, the Fourth Huntsville Gamma Ray Burst Symposium drew more than 200 top astrophysicists - observers and theorists alike - to offer and critique data and theories.