Discovery May Be "Smoking Gun" In Gamma Ray Burst Mystery

March 31, 1997

An international team of University and NASA astronomers have detected a flash of light from a distant galaxy -- a sentinel that may definitively solve a 30-year mystery in astrophysics by demonstrating that cosmic gamma-ray bursts come from the distant reaches of the universe.

That would mean gamma-ray bursts represent an energy release in a few seconds equivalent to the amount our Sun will emit in its entire ten-billion-year lifetime.

A team led by Dr. Jan van Paradijs, the Pei-Ling Chan eminent scholar in physics at The University of Alabama in Huntsville and a professor of astronomy at the University of Amsterdam, has identified a dim, distant galaxy as the possible source of a gamma-ray burst that swept through the sky on February 28.

Working from the Space Sciences Laboratory at NASA's Marshall Space Flight Center, van Paradijs and his international team used data from three satellites, two optical observatories and a radio observatory in their attempt to pinpoint the burst's origin.

His team includes Dr. Chryssa Kouveliotou of the Universities Space Research Association at Marshall, Dr. Richard Strom at Westerbork Radio Observatory in the Netherlands, and graduate students Paul Groot and Titus Galama at the University of Amsterdam.

The detailed results of their reseach have been accepted for publication in the scientific journal, Nature. These findings could force many astrophysicists to rethink their theories about the origins of these violent explosions of energy.

One of the least understood phenomena in astrophysics, gamma-ray bursts are powerful flashes of gamma rays that are detected about once a day from random locations in the sky. Gamma rays are electromagnetic radiation at the highest end of the energy spectrum, carrying energies almost a million times higher than visible light.

When they happen, gamma-ray bursts outshine all other sources of gamma rays combined.

The discovery of gamma-ray bursts in the late 1960s was accidental. Since that time, astronomers who tried to find the source of gamma-ray bursts found nothing unusual when they looked in the directions from which bursts originated.

Competing theories, developed using data from NASA's orbiting Burst and Transient Source Experiment (BATSE), place the bursts' origins either just outside the Milky Way galaxy in a large spherical halo, or from remote parts of the universe at what astronomers call "cosmological distances."

On Feb. 28, the Gamma-Ray Burst Monitor aboard the Italian-Dutch BeppoSAX satellite detected a gamma-ray burst that was also within the field of view of one of the SAX Wide Field Cameras. Built by the Netherlands Space Research Organization (SRON), these cameras can pinpoint in the sky the position of a burst within a circle roughly one-fifth the diameter of the Moon as seen from Earth.

Gamma-ray burst detectors aboard the Ulysses and Wind spacecraft also saw the burst.

"With the location narrowed to that small area, we were able to obtain a few optical images of that region of the sky with the 4.2 meter William Herschel telescope of the La Palma Observatory in the Canary Islands," van Paradijs said. "We took our first image just 21 hours after the burst was detected, and the second about a week later. We were astounded by what we saw."

In the first image, van Paradijs and his team found a light source that does not appear on the second, apparently a much-sought-after and almost legendary "optical transient."

Within eight hours after the burst was detected, the BeppoSAX spacecraft was maneuvered to point its more precise X-ray imaging instruments at the location indicated by the Wide Field Camera. The X-ray detectors saw a glowing X-ray source, 5,000 times fainter than the Crab Nebula. Its position was narrowed to within one arc minute (one-sixth the diameter of the original box), consistent with the source of the gamma-ray burst.

Three days later, the X-ray source was virtually gone, dropping in brightness by nearly a factor of 20.

"The X-ray results by themselves are already very exciting," said van Paradijs. "But now that we also have an optical transient, we can nail its position to better than one arc second (one sixtyeth of the one-arc-minute box)."

"What is most interesting," said Kouveliotou, "is that an image taken March 13 by the New Technology Telescope at the European Southern Observatory in Chile found, in nearly the exact spot where the optical transient appeared in the first photograph, a very faint galaxy."

The locations of the optical transient and the galaxy are separated by at most one-quarter of an arc second - the width of a quarter viewed from a distance of more than 10 miles.

"To me, this is fairly convincing evidence that the transient X-ray and optical sources are the same, and that both are associated with the gamma-ray burst," said van Paradijs. "If these transients are from the distant galaxy, we have, for the first time, found the site of a gamma-ray burst."

Astronomers do not know exactly how far away the faint galaxy is, but they do know it is distant. The Hubble Space Telescope has been pressed into duty to take a look.

"Hubble should have about a one hundred-or-so pixel elements from which to make an image, so we should get a very nice picture," said van Paradijs.

"Most researchers in the field, based largely on the BATSE data, were already leaning toward the cosmological hypothesis," said Dr. Charles Meegan, a BATSE co-investigator and gamma-ray burst expert at Marshall. "But this detection might even convince the remaining skeptics."

NASA/Marshall Space Flight Center News Center

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