December 17, 1996

updated: January 3, 1997

The ongoing scientific debate over the nature and distance to cosmic gamma-ray bursts is sure to heat up further following the latest discovery by Space Sciences Laboratory scientists at NASA/Marshall Space Flight Center.

Using the Burst and Transient Source Experiment (BATSE), a NASA/Marshall experiment on NASA's Compton Gamma Ray Observatory, astronomers have detected a unique sequence of four cosmic gamma-ray bursts, unlike any seen to date in the 5.5 year mission. Other spacecraft have confirmed several of these blasts of energy, which appeared in rapid succession over a two day period from the same general position in the sky.

Scientists will be giving a full account of their findings on December 18, 1996 in a paper presented to the Texas Symposium on Relativistic Astrophysics, held in Chicago, Illinois.

 

BATSE usually detects only about one gamma-ray burst per day, and the locations in the sky of nearly 1,700 events detected to date (diagram) appear to be randomly distributed. "That's what makes these four events so unusual," says Dr. Charles Meegan of NASA/Marshall, and a co-investigator on the BATSE experiment. "They came right after one another, about two days apart, and all from the same part of the sky. If you make a circle with your thumb and index finger and hold it out at arms' length, that about covers the area on the sky where these bursts came from."

Although that may seem like a fairly small region of the sky, it's many times too big to be searched efficiently by instruments such as the Hubble Space Telescope, because their field of view is significantly smaller. These telescopes would have to fill in the area one small square at a time, taking thousands of pictures to build a mosaic of this region.

Because of the uncertainty in the events' locations, and because of the possibility of a chance coincidence, the BATSE astronomers can't say for sure that these events were produced by just one object in space. "Stranger things have happened," said Dr. Valerie Connaughton, a National Research Council scientist at NASA/Marshall working on BATSE data, and principal author on the paper, "but it would be somewhat unlikely that this actually happened by chance."

In fact, the scientists are not certain that these events are all from the same object, and there is even some discussion about how many bursts actually occurred. "The instrument triggered four times," Meegan explained, "but at least two of the gamma-ray peaks appear to be connected in the same event. So even if we can determine for certain that these gamma-rays came from the same object, there's the next question of whether it's one long burst, stretching over two days, if it's actually a repetition of individual bursting events, or something in between. If it turns out to be one long burst, then they're even stranger than we thought."

 

NASA/Marshall's Dr. Gerald Fishman, principal investigator on BATSE agreed. "It's trying to tell us something about what causes these bursts, but we're not sure what just yet." The distance to gamma-ray bursts is still debated among scientists, despite nearly 25 years of study. While some astronomers hold out for an explanation that puts the bursts just outside of our own galaxy, most believe that the bursts come from remote parts of the universe, at cosmological distances of a billion light years or more. "But some cosmological models have problems with a repeating event," Fishman continued, "because they usually involve a tremendous explosion that destroys the source in the process. You just can't come back two days later and make another burst."

The dates and times (in seconds of day Universal Time, or UT) of the burst detections are shown in the table below, along with an indication of any simultaneous detection by the Ulysses, TGRS, and Konus gamma-ray burst experiments. The computed position of each event on the sky is shown in the coordinates of Right Ascension (RA) and Declination (DEC), with the uncertainty in each position given in the column labeled "Uncert." The duration of each event is also listed below in seconds.

Click for a sky location map of the bursts. Click the times below for a profile of each burst.

 

 

 

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