Gamma-ray bursts used to be a rather esoteric field of astrophysics. Bursts were interesting, partly because they were so elusive. But they were a quirk that didn't command the same attention as bigger, better-known phenomena. Today they often command significant observing time by the Hubble Space Telescope and the Keck Observatory, the two optical telescopes most in demand by astronomers.
Observations and discoveries over the last three years have made gamma-ray bursts front page news, reshaping our perception of how they fit into the grand scheme of the universe.
Later this month, more than 200 scientists will gather to discuss their findings and their plans for unraveling more about these mysterious bursts of energy.
"The exciting thing is that we will have scientists from lots of different astronomical fields, not just high-energy astrophysics," said Dr. Valerie Connaughton, a member of the organizing committee for the 5th Huntsville Gamma Ray Burst Symposium. "We also have optical [visible light] and radio astronomers planning to attend."
Connaughton is an astrophysicist working at NASA's Marshall Space Flight Center. The 5th Huntsville Gamma Ray Burst Symposium will be held Oct. 18-22 at the Huntsville Hilton Hotel under the sponsorship of NASA/Marshall and the Universities Space Research Association. Gamma-ray bursts have puzzled scientists since they were discovered in the late 1960s by satellites watching for nuclear weapons tests in space. They recorded bursts of radiation in gamma rays, the highest part of the electromagnetic spectrum, but not lower down. And the bursts appeared to be coming from outside the solar system. Their appearances and locations were random and not associated with any known object.
As other spacecraft observed bursts and the puzzle deepened, NASA decided to include a dedicated instrument, the Burst and Transient Source Experiment, on the Compton Gamma Ray Observatory.
"BATSE would detect bursts with unprecedented sensitivity so that by studying their distribution in the sky, we could establish their origins in our galaxy," Connaughton said of early hopes for BATSE.
Instead, scientists got an even deeper mystery. Bursts flashed and faded in a matter of seconds or minutes, too quick to aim a telescope for follow-up observations. And they appeared to be distributed outside the galaxy and probably deep in the universe.
The big break came in 1997 when Dr. Jan van Paradijs of the University of Amsterdam, using observations by the Beppo SAX satellite and ground-based observatories, tied a burst on Feb. 28, 1997 to a source deep in space. The Feb. 28 burst was the big news at the 4th Huntsville Gamma Ray Burst Symposium in September 1997.
"The sense of the community is that the doubt is over," said Dr. Chip Meegan, a BATSE coinvestigator at NASA/Marshall, before the 1997 symposium. "Gamma ray bursts are cosmological." That means that instead of coming from within our galaxy or even immediately around the galaxy, they are deep in space, probably more than 8 billion light years away (by comparison, our galaxy is about 150,000 light years across).
"But they're still very strange," Meegan continued. "All of the questions about them being cosmological are still there." The principal question is, what produces so much energy?
The questions still stand and will be discussed, along with findings from the past two years of observations at the 5th Huntsville Gamma Ray Burst Symposium.
"The supernova crowd will be there since we have a tentative association between supernovae and gamma-ray bursts," Connaughton said. "There's been lots of speculation by astronomers active in supernova research as well as those active in gamma-ray bursts."
If the two are associated, then what special conditions lead to a supernova expending such a phenomenal amount of energy mainly in the gamma-ray spectrum yet hiding or muting itself in visible light until weeks later?
In January, the Robotic Optical Transient Search Experiment at Los Alamos National Laboratory, cued by BATSE, caught the optical flash within 20 seconds of a gamma-ray flash being recorded. But many other attempts to catch optical transients coincident with the gamma-ray emissions have been fruitless.
"There is a really intense debate as to whether these optical flashes happen with all bursts," Connaughton noted.
Another topic that has grown enough to warrant its own discussion is Soft Gamma Repeaters (SGRs). Where true gamma-ray bursters are distant and never repeat (the blast is so energetic that it shreds the source object), SGRs are within our galaxy and repeat at unpredictable times.