June 9, 1998: One of the signs that a scientific discipline has arrived is when a major science conference devotes an entire session to a topic. This week's annual meeting of the American Astronomical Society (AAS) will feature a session on "Gamma-ray Burst Counterparts and Afterglows," scheduled for today.
As late as two years ago the session would have been impossible because no one had found a counterpart. The catalog had more than 1,500 burst events, most recorded by the Burst and Transient Source Experiment (BATSE) on the Compton Gamma Ray Observatory. But the burst were so transient that no one had been able to pinpoint a location or see one with other instruments. That changed on Feb. 28, 1997, when the Beppo-SAX satellite happened to be looking in the right direction when a burst went off.
Now burst counterparts and the afterglows from them are being discovered at a pace that might seem leisurely.
"We still have only a few," said Dr. Gerald Fishman, BATSE principal investigator at NASA's Marshall Space Flight Center. "We have five or six optical counterparts, nine or ten in X-rays, and two in radio."
A counterpart means an object seen by one telescope in the same sky location and behaving in much the same manner as an object seen by another telescope looking at the same piece of sky. In modern astrophysics, it is important to examine objects in all parts of the spectrum - from radio through visible up to gamma rays - to fully account for what a star, galaxy, or other object is doing.
In the case of gamma-ray bursts, finding a counterpart in another part of the spectrum was crucial because the bursts were so bright and brief that they led to a lot of theories but few conclusions.
The Feb. 28, 1997 burst - known as GRB 970228 - was the first to be seen glowing afterward in X-rays and visible light. It soon became apparent that it was associated with an fuzzy object that may be an extremely distant galaxy.
"A more recent counterpart (Dec. 14, 1997) started the theorists really scratching their heads because the source was too bright to match the preferred model of two neutron stars colliding," Fishman said.
On Tuesday at the AAS, Fishman will chair the session on counterparts and afterglows. Fourteen papers are scheduled and will cover the electromagnetic spectrum from radio through gamma rays.
"I expect a pretty good crowd," Fishman said. "A year ago we wouldn't have had enough speakers to apply for a special session." The session includes speakers from Utrecht and Amsterdam, The Netherlands; Rome and Bologna, Italy; and several U.S. universities and NASA/Marshall. Included are scientists who use the most powerful ground-based telescopes, the Keck Observatory in Hawaii and the Very-Large Array at the National Radio Astronomy Observatory in New Mexico.
"All of the top people who have been making radio, optical, and X-ray observations of gamma-ray burst counterparts are going to be speaking," he said. "I expect there will be some new findings from the speakers. It's really an exciting time in this field."