 Space Science News home |
|
|
The stage was set when the Compton Gamma Ray Observatory was launched by the Space Shuttle Atlantis (STS-37) on April 5, 1991. The crew deployed it on April 8 (after helping unstick an antenna). A few days later, the spacecraft started observing the universe with a sensitivity never before achieved. Right: Circles indicate the locations of four of eight BATSE instruments on the Compton Gamma Ray Observatory. The other four are located directly on the opposite side of the spacecraft. Aligned down the center are the other three instruments (from top) are: Oriented Scintillation Spectrometer Experiment (OSSE), Imaging Compton Telescope (COMPTEL), Energetic Gamma Ray Experiment Telescope (EGRET). (See box below)
Left: The first hints that things weren't as astronomers assumed. This is a thumbnail from a sky map on which Dr. John Horack, a BATSE team member, recorded the first 60 bursts detected by BATSE. Clicking on the image will show the full map (600x354 pixels, 132K JPG) with the bursts distributed randomly across the sky. Compare it with the sky map of 2,000 bursts near the en o this story. "BATSE changed everything in the fall of 1991 when we announced that gamma-ray bursts were evenly spread across the sky and there was a deficit of weak bursts," Fishman explained. "It's completely changed our views of what gamma-ray bursts are."
Since the late 1960s when they were discovered by satellites watching for violations of a nuclear test ban treaty, gamma-ray bursts have puzzled scientists. Detectors on the monitoring satellites, and later aboard interplanetary spacecraft could not resolve the puzzle. (Three sources were located and later identified as Soft Gamma Repeaters, and are now believed to be magnetars, a totally different kind of object than a 'classical' gamma-ray burst. But most of the bursts had higher energies and no known location.) When NASA started planning the Compton Gamma Ray Observatory, Fishman proposed including BATSE as a burst alarm. It would quickly tell where a burst was located so the three larger instruments could pin them down and solve the mystery. [see box] Fishman, Meegan, a co-investigator at NASA/Marshall, and other scientists expected that BATSE would show most of the bursts were clustered along the Milky Way, the bright band across the heavens where most of our galaxy's stars are located. Instead, the bursts were scattered across the sky "This was inconsistent with any known distribution of objects in our galaxy," Fishman said. "That immediately caused a significant fraction of the scientific community to rethink the galactic neutron star model of bursts, and start thinking about cosmological models." Fishman described presenting his finding in October 1991 to a GRO science meeting: it was total silence "followed by a series of rapid-fire questions. It was an electrifying experience. Once in a lifetime." But the change wasn't universal. Bursting with energy Fishman admits getting a slight knot in his stomach, and not just because he was about to upset a cherished theory. "All that summer I got a queasy feeling because I realized the significance of what we were finding," he continued. "It actually scared me somewhat because I realized these are the largest known explosions in the universe." The implication was that they were incredibly distant and therefore incredibly bright. Since then, scientists have continued to debate possible sources, and BATSE continued recording about 8 new bursts every 10 days. In late 1997, it reached No. 2,000. Last week, the count was near 2,400. Almost four times as many bursts have actually happened in that time, since the Earth blocks almost half the sky from BATSE, and other factors cut in half the number of bursts that BATSE can detect. It was one of those "over-the-horizon" bursts that helped nail down the distance to bursts. On Jan. 23, 1997, Italy's Beppo SAX X-ray astronomy satellite was pointed in the right place at the right time. Ironically, CGRO was just 5 minutes below the horizon and missed the show. But a scientist affiliated with the BATSE team was able to use time on an optical telescope to use Beppo SAX data and pin the burst to a distant galaxy that then was studied by the Hubble Space Telescope. Chance is less of a factor now with the addition in 1997 of the University of Michigan's Robotic Optical Transient Source Experiment (ROTSE) that takes a cue from BATSE and quickly points an array of four cameras in the right area. On. Jan. 23, 1999, it captured the first optical images of a burst "in the act" just 20 seconds after BATSE was triggered, and while the burst was still emitting gamma-rays. Yet even with this wealth of data, scientists still aren't sure what causes the bursts. Initially, they thought that the bursts were caused by two neutron stars orbiting each other and eventually colliding, or a neutron star being swallowed by a black hole. Little else could account for a blast that emits more energy in a few seconds that our sun will emit in its entire lifetime. None of the theories solved all of the problems, including going directly to gamma-ray emissions. Now scientists are studying the hypernova model. This involves a star about 50 times as massive as our sun reaching the end of life. It has burned everything its core, implodes and then explodes. It seems to be the only way of producing a fireball whose shock front moves near the speed of light and generates the immense quantities of energy required. It would also appear to be something common in the early universe, but far less likely in older, closer galaxies. Gamma-ray bursts could be closely linked to our own lives since the dust that became planets was forged in the hearths of explosions like supernovas and hypernovas. |
April 5, 1999: They didn't
know they were about to do it, but eight years ago scientists
led by a team of astronomers in Huntsville, Ala., were on the
verge of discovering the incredible. What appeared to be local
firecrackers in the sky, were instead distant megablasts, each
releasing more energy in ten seconds than the Sun emits in its
entire 10-billion-year lifetime. It was the last thing they expected
to find from these accidentally-discovered cosmic enigmas.
"This is something
I'm really proud of," Fishman said. "These are some
of the brightest students I've ever seen."
Join
our growing list of subscribers - 
Headlines