A whopper of a discovery

A powerful gamma-ray burst on Jan. 23 was briefly visible through binoculars and amateur telescopes

January 27, 1999: Cosmic gamma-ray bursts have been called the greatest mystery of modern astronomy. They are powerful blasts of gamma- and X-radiation that come from all parts of the sky, but never from the same direction twice. Space satellites indicate that Earth is illuminated by 2 to 3 bursts every day.

What are they? No one is certain. Until recently we didn't even know if they came from the neighborhood of our own solar system or perhaps from as far away as the edge of the universe. The first vital clues began to emerge in 1997 when astronomers detected an optical counterpart to a gamma-ray burst. In February of that year the BeppoSAX X-ray astronomy satellite pinpointed the position of a burst in Orion to within a few arcminutes. That allowed astronomers to photograph the burst, and what they saw surprised them. They detected a rapidly fading star, probably the aftermath of a gigantic explosion, next to a faint amorphous blob believed to be a very distant galaxy.

The Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory, pictured above, has recorded over 2000 cosmic gamma-ray bursts since it began operations in 1991.

This image from the Hubble Space Telescope shows the optical afterglow from a gamma-ray burst detected in February 1997. The bright spot is thought to be an expanding fireball, and the weak diffuse emission (below and to the right) may be the distant host galaxy.

Shri Kulkarni of Caltech and his colleagues found that a gamma-ray burst recorded in December 1997 came from a faint galaxy with a redshift of 3.4. That means that the burst originated over 12 billion light years away. Kulkarni noted that "The energy released by this burst in its first few seconds staggers the imagination." Indeed, it was one of the biggest explosions since the Big Bang itself.

Since then seven more optical counterparts have been discovered, but until last week the fiery afterglows were captured on film only after the gamma-ray burst was already over. Gamma-ray bursts occur with no warning and typically last for just a few seconds. By the time astronomers trained their optical telescopes in the direction of the burst, the action was over.

On Jan. 23 astronomers captured the first optical photographs of a gamma-ray burst while the burst was still underway. The gamma-ray-burst detectors of the Burst and Transient Source Experiment (BATSE) onboard NASA's orbiting Compton Gamma Ray Observatory detected the beginning of a bright gamma ray burst at 4:23 a.m. Eastern Standard Time. As the burst was still in progress, computers determined a rough location and radioed the position to the Gamma Ray Burst Coordinates Network (GCN), based at NASA's Goddard Space Flight Center, Greenbelt, MD. The position was immediately forwarded by email to astronomers at ground based observatories throughout the world.

Just 22 seconds later the Robotic Optical Transient Search Experiment (ROTSE) in Los Alamos, NM, operated by a team led by Dr. Carl Akerlof of the University of Michigan, was in position and took images of the patch of sky where the burst was reported. Their equipment is assembled from 35 mm camera lenses and parts culled from the amateur astronomy market. The first picture showed a brightening new star within the sky region where the burst was reported.

Five seconds later the burst achieved peak brightness, reaching 9th magnitude, about 16 times fainter than the human eye can see, but easily visible in an amateur telescope. Within eight minutes of the initial detection, the burst had faded by a factor of 100 below its maximum brightness. "I was amazed," Akerlov said. "At best, we expected something really dim optically, at the limit of our sensitivity. Instead we found a whopper."

Right: The gamma ray and optical counterpart burst is near the constellation Corona Borealis. In late January, Corona Borealis transits around 7 a.m. local time in the northern hemisphere. The best time to view this part of the sky is in the pre-dawn hours between 4 and 5 a.m.when it will be high in the southeast. A heavenly object "transits" when it passes through a meridian, an imaginary line in the sky drawn from north to south and directly observer's head.

"If this burst had originated in the Milky Way Galaxy, it would have lit up the night sky," said Dr. Alan Bunner, Director of NASA's Structure and Evolution of the Universe science theme at NASA Headquarters.

Most experts believe that optical counterparts to gamma-ray bursts hold the key to understanding these mysterious cosmic explosions. The first discovery of visible fireballs in 1997 resulted in convincing proof that gamma-ray bursts take place in the distant reaches of the universe. Now that we know where gamma-ray bursts come from -- very far away -- the next daunting task is to understand what causes them. In the absence of much hard data theorists have proposed a multitude of possible scenarios, from super-supernovae to mutually annihilating neutron stars. Light curves obtained during gamma-ray explosions should help astronomers distinguish between competing theories, and just perhaps solve astronomy's "greatest mystery."

Web Links
More science headlines - NASA space science research Blasts from the Past - a review of recent gamma-ray burst discoveries Cosmic Gamma Ray Bursts - from Science@NASA	Magnetars.org - all about magnetars and soft gamma-ray repeaters BATSE Home Page The Synchrotron Shock Model confronts a 'Line of Death' - PDF preprint of the Astrophysical Journal Letter. Reference: Preece et al. 1998, ApJ, 506, L23

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Author: Dr. Tony Phillips
Curator: Bryan Walls
NASA Official: John M. Horack