Search For Counterparts, Spectral Features, And Clues Highlight 2nd Day Of 4th Huntsville Gamma Ray Burst Symposium

September 18, 1997

If you've seen one, you haven't seen 'em all. Or even a lot of them. Great excitement followed the discovery of optical counterparts for GRB 972028, the February 28 gamma ray burst, (pictured at left, and GRB 970508, and X-ray counterparts for a few others. But the counterparts for more than 1,900 bursters remain hidden despite intensive searches, and nature is releasing few clues to how to find them.

"The facts we have in hand refuse to organize themselves into any kind of coherent framework," said Dr. Kevin Hurley of the University of California at Berkeley. "We have not observed any flaring radio or optical counterparts, but have in X-rays. We have observed fading counterparts in radio, optical, and X-rays."

Hurley reviewed the search for counterparts to bursts at the Fourth Biennial Huntsville Gamma Ray Burst Symposium hosted by NASA's Marshall Space Flight Center. On Thursday, scientists will discuss their searches for counterparts in visible and radio wavelengths, and discuss what kinds of galaxies might be home to these most elusive phenomena.

Although not finding something means failure to most people, to scientists it just means an upper limit, a brightness or strength that an event will not reach, so you have to look closer or use a more powerful instrument to find the details.

Before the Dutch-Italian Beppo-Sax satellite discovered the February 28 burst, several searches with radio and optical telescopes turned up nothing that could be matched to a gamma ray burst. However, radio, optical, and X-ray counterparts have been found for a few fading bursts.

Japan's Advanced Satellite for Cosmology and Astrophysics and the United States' Rossi X-Ray Timing Explorer have seen fading X-ray components of at least three bursts. In the case of GRB 970616 (June 16), ASCA saw three sources within the error box defined by other satellites. One is the likely GRB counterpart because it is fading rapidly.

"All the ASCA observations are fading rather quickly," said Toshio Mirukami of Japan's Institute of Space and Astronautical Sciences.

Luigi Piro and Enrico Costa of Italy's Institute of Astronomical Sciences said Beppo Sax has discovered three X-ray afterglow counterparts, but that they also faded rapidly, and in the case of the May 8 event, the spectra changed rapidly, either due to changes in the burst's own brightness or because of absorption by material between the burst and Earth.

Y. Takeshima of Goddard Space Flight Center said that the Rossi X-Ray Timing Explorer has only been able to catch two of five bursts it was tasked to locate.

"Different bursts with different properties will have different fading counterparts," Hurley said. "So how do we organize the bursts (so we can set up better searches for counterparts)? I don't know the answer to this one. I'm not even sure there is one correct answer."

Part of the challenge is that so few counterparts have been found that no pattern can emerge. The two optical counterparts are "not terribly consistent." GRB 972028 appears to be deep in the universe and embedded in the edge of a fuzzy galaxy. GRB 970508 appears to be in a slightly closer galaxy about 8 billion light years away and has a radio counterpart; none has been detected for GRB 970228. Although in gamma rays these two resemble many other bursts, that similarity has not led to the discovery of other fading bursts.

Nor has anyone definitely found quiescent counterparts, the nearly dead, still glowing aftermath of an old burst that still would be worth studying.

Hurley said that about 100 counterparts are needed, then "we could really do something about the nagging questions."He expects that continued observations with Beppo Sax, and with the High Energy Transient Experiment (scheduled for launch in 1999) will locate more X-ray counterparts and help clarify the issue.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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