Radio Observations Provide New Clues To Nature Of Gamma Ray Fireball

September 17, 1997

A team of astronomers using a pair of National Science Foundation (NSF) radio telescopes has made the first measurements of the size and expansion of a mysterious, intense "fireball" resulting from a cosmic gamma ray burst last May.

After three months of observations with the NSF's Very Large Array (VLA) and Very Long Baseline Array (VLBA) radio telescopes, scientists have learned that the "fireball" of debris expands very closely to the speed of light. They estimate its current size to be about one-tenth of a light-year, or 170 times the distance between the sun and Pluto. The scientists reported their findings about the May 8 gamma ray burst in the September 18 issue of Nature.

"For 30 years, we've known almost nothing about these mysterious explosions in the sky. Our observations show that these events release truly incredible amounts of energy," said Dale Frail, of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico.

"It was only a few months ago that our observations showed that such bursting objects are located far beyond our own galaxy. However, astronomers had little evidence for how this cosmological juggernaut actually works. The radio observations have revealed a size of the fireball, unobtainable by any other technique, thereby enabling astronomers to learn about inner workings of such objects," said Shri Kulkarni, professor of astronomy at the California Institute of Technology (Caltech).

In addition to Frail and Kulkarni, the astronomers are Greg Taylor of NRAO and Italians Luciano Nicastro and Marco Feroci of the BeppoSAX Gamma Ray Burst Team. BeppoSAX is an Italian-Dutch satellite, launched late last year, that detects gamma ray bursts and provides precise sky positions to allow ground-based telescopes to observe them.

Cosmic gamma ray bursts, occurring about once per day, have been observed for some 30 years. However, until this year, very little was known about them. Even their distances from Earth were the subject of great debate among astronomers.

This year, the rapid and improved positions provided by the BeppoSAX satellite have allowed astronomers to look for the bursts, or their optical/radio counterparts, quickly with other instruments on the ground and in space. As a result, two bursters, including the one on May 8, have been detected optically. The VLA found radio emission from the May 8 burster on May 13, and the VLA and VLBA have been monitoring the object regularly since then.

"Shri (Kulkarni) and I searched for radio counterparts to gamma ray bursters for four years, but only when BeppoSAX provided us with accurate positions were we able to identify one," said Frail.

Gamma ray bursts, the causes of which still are unknown, are short-lived phenomena. The May 8 burst lasted only 15 seconds. However, X-ray, optical and radio emission continues in an "afterglow." It is that afterglow that is revealing details of the fireball created by the initial explosion. That initial explosion released, in 15 seconds, more energy than the sun will release in its entire, 10-billion-year lifetime.

Optical studies of the May 8 object using the 10-meter W.M. Keck Telescope in Hawaii indicated that it is very distant, at least seven billion light-years away.

While the burst's afterglow showed a steady decline in brightness at both optical and X-ray wavelengths, that was not the case at radio wavelengths. The radio emission rose and fell several times, and the relative intensity at different radio wavelengths also changed.

While puzzling over this behavior, Frail and Kulkarni learned that Jeremy Goodman of Princeton University already was predicting this effect. Irregularities in the extremely tenuous material between the stars could cause fluctuations in the radio intensity as seen from Earth. "As we collected more data, it became clear to us that indeed the radio object 'twinkled,'" Kulkarni said.

"Many amateur astronomers know that stars twinkle, but planets don't," Frail explained. "This is because the stars are so distant they appear as mere points, and irregularities in the atmosphere cause them to twinkle. Planets, on the other hand, are close enough that they are not mere points, and their larger apparent size squelches the twinkling. Over three months, we saw the radio twinkling of the gamma ray burster slowly cease. From this we could calculate both its apparent size and the rate of its expansion."

"It is interesting to note that the irregularities in the thin material between the stars enable us to obtain information unobtainable any other way. Without this effect, we would need a radio telescope array the size of the sun to measure this object," said Greg Taylor, who led the effort to observe the object with the VLBA.

Using the ultra-sharp "vision" of the continent-wide VLBA, the astronomers have pinpointed the burster's position in the sky with extremely high precision -- less than a thousandth of a second of arc. In the three months of their observations, they have noted that this position has not changed measurably, thus strengthening the case for the object's being at a great distance.

The VLA and VLBA are instruments of the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.


National Science Foundation

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