Twinkle, Twinkle, Massive Fireball

September 19, 1997

The twinkling of a radio source deep in space has let astronomers calculate that at least one gamma ray burst has expanded to 85 times the size of our solar system and is still growing.

Dr. Dale Frail and Dr. Greg Taylor of the National Radio Astronomy Observatory today disclosed the results of the first radio astronomy observations of a gamma ray burst. The numbers, quite simply, are huge.

When a star explodes - goes supernova - it can be seen across the entire electromagnetic spectrum. Usually, a supernova is first noticed in visible light, and then in radio waves and X-rays as astronomers train more and more telescopes on the object. In the case of gamma ray bursts, almost nothing was turned up in nearly 30 years of searching. The first optical counterpart was found in February of this year (GRB 970228), and in May the first radio counterpart was discovered.

On May 8, the Dutch-Italian Beppo Sax satellite observed a gamma ray burst, and provided a good location. Just 3.9 hours later, telescopes in the Very Long Baseline Array were looking for the burst.

The VLBA actually is more than a dozen radio telescopes from the Virgin Islands to Hawaii. The telescope at St. Croix, the Virgin Islands, is pictured above. Their observations can be electronically linked so their resolving power is the same as a single telescope almost as wide as the Earth. The collecting power is still quite low and takes several days to build an image.

"On May 13, we found the radio source sitting right on top of the optical source (which other telescopes had located)," Frail said.

What they saw was not the smooth power law decay that has come from observations with the Hubble Space Telescope, but erratic fluctuations for about three weeks. Then they dampened and over the next 80 days have gradually smoothed and seem to be approaching the power law.

Frail explained that what happened was the equivalent of stars twinkling, and that the point where the twinkling stops gives them a way to measure the diameter of the fireball.

"This is a common thing in radio astronomy, equivalent to optical seeing," he said. "Seeing" is just that, how well you can see the stars. Light twinkles when it passes through a turbulent medium where the waves are larger than the beam of light from the object you are observing, and when the medium can interact with the light. Thus, astronomers say that "stars twinkle, planets don't."

In the case of GRB 970508, the burst twinkled until it was larger than the turbulence in the interstellar gas. The twinkling actually goes on, but over such a large area that it largely cancels itself.

Based on the apparent diameter of the star and the wavelength at which it was observed, Frail calculated that after 80 days, the fireball's diameter is 10 trillion kilometers (6.2 trillion miles), or 85 times the diameter of the solar system - more than 600,000 times the volume. That is double its size a week after the blast. Taylor said the source is compact and is not linked to any extended objects, so it does not appear to be related to certain types of galaxies which had been candidates for burst sources.

As with optical counterparts, radio counterparts of gamma ray bursts are elusive. Frail said that in 17 years of searching for radio counterparts, nothing has turned up.

"The boxes are largely empty," Frail said, referring to the small sections of sky where other instruments say the bursts are located."Clearly, radio counterparts of gamma ray bursts are rare," Frail said. But he also noted that the low detection rate is similar to the rate for finding supernovae, a far more common occurrence.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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