• A section of the Hubble Space Telescope image of the field of GRB 990123, showing the afterglow and its host galaxy, taken about 16 days after the burst. The images were processed at Caltech by Dr. Stephen Odewahn, a member of the Caltech team.
    Credit: The Caltech GRB Team/Space Telecsope Science Institute
  • A comparison of images obtained at Palomar Observatory. The image on the top is from the Palomar Observatory's digital sky survey (DPOSS). The image on the bottom is the discovery image obtained by S. C. Odewahn and J. S. Bloom.
03/25/1999 08:00:00

Caltech observes brightest gamma-ray burst so far

PASADENA-An extraordinarily bright cosmic gamma-ray flash turns out to be the most energetic one measured so far, according to a team of astronomers from the California Institute of Technology.

"The burst appeared to be more luminous than the whole rest of the universe, and that would be very hard to explain by most current theories,"said Caltech professor of astronomy and planetary science Shrinivas Kulkarni, one of the principal investigators on the team.

"It was ten times more luminous than the brightest burst seen so far, and that was quite unexpected."

"If the gamma rays were emitted equally in all directions, their energy would correspond to ten thousand times the energy emitted by our sun over its entire lifetime so far, which is about 5 billion years," said Caltech professor of astronomy S. George Djorgovski, another of the principal investigators on the team. "Yet the burst lasted only a few tens of seconds."

Gamma-ray bursts are mysterious flashes of high-energy radiation that appear from random directions in space and typically last a few seconds. They were first discovered by U.S. military Vela satellites in the 1960s. Since then, over a hundred theories of their origins have been proposed, but the causes of gamma-ray bursts remain unknown. Some theorists believe that the bursts originate during the formation of black holes.

NASA's Compton Gamma-Ray Observatory satellite has detected several thousand bursts so far. The chief difficulty in studying these puzzling flashes is in locating them precisely enough and quickly enough to follow up with ground-based telescopes.

A breakthrough in this field was made in early 1997 by the Italian/Dutch satellite BeppoSAX, which can locate the bursts with a sufficient accuracy. A team of Caltech astronomers was then able to establish that the bursts originate in the very distant universe. Since then, about a dozen bursts have been studied in detail by astronomers using ground-based telescopes.

The bursts may last only a few seconds in gamma rays, but leave more long-lived but rapidly fading afterglows in X-rays, visible light, and radio waves, which can be studied further.

This burst, called GRB 990123, was discovered by the BeppoSAX satellite on January 23. It was the brightest burst seen so far by this satellite, and one of the brightest ever seen by NASA's Compton Gamma-Ray Observatory.

Within three hours of the burst, members of the Caltech team, including senior postdoctoral scholar in astronomy Stephen Odewahn and graduate students Joshua Bloom and Roy Gal, used Palomar Observatory's 60-inch telescope to discover a rapidly fading visible-light afterglow associated with the burst.

"This adventure began at 5 a.m. with a wake-up call from our Italian friends alerting us about their burst detection," said Bloom, "But it was certainly worth it. We got to watch a remarkable fireworks show!"

A comparison of images obtained at Palomar Observatory. The image on the top is from the Palomar Observatory's digital sky survey (DPOSS). The image on the bottom is the discovery image obtained by S. C. Odewahn and J. S. Bloom.

Following the Caltech team's announcement, several hours later a team of astronomers known as the ROTSE collaboration, led by Professor Carl Akerloff of the University of Michigan, reported that the visible light counterpart of the burst was also seen in the images taken with a small, robotic telescope operated by their team, starting only 22 seconds after the burst. This was the first time that such rapid measurement of a burst afterglow was made, and its extreme brightness was unexpected.

Meanwhile, a new radio source, coincident with the visible-light afterglow discovered at Palomar, was found at the National Radio Astronomy Observatory's Very Large Array radio telescope, near Socorro, New Mexico, by Dale Frail and Kulkarni.

Such a radio flash was predicted by Dr. Re'em Sari, a theorist at Caltech, and Dr. Tsvi Piran (now at Columbia University), and it provides an important input for theories of gamma-ray bursts.

At the prompting of the Caltech team, a group of astronomers led by Professor Garth Illingworth of the University of California at Santa Cruz, used the W. M. Keck Observatory's 10-meter Keck-II telescope at Mauna Kea, Hawaii, to obtain a spectrum of the burst afterglow.

A distance to the burst was determined from its spectrum, and the burst was found to be about 9 billion light-years from Earth.

The Keck measurement of the distance was crucial. "We were stunned," said Djorgovski. "This was much further than we expected, and together with the observed brightness of the burst it implied an incredible luminosity.

"The peak brightness of the visible light afterglow alone would be millions of times greater than the luminosity of an entire galaxy, and thousands of times brighter than the most luminous quasars known."

This remarkable light flash contained only a small fraction of the total burst energy in the gamma rays. Caltech astronomers note that even more energy was likely emitted in forms that are difficult to observe, such as gravitational waves or neutrinos, elusive particles that can penetrate the entire planet Earth without stopping.

As the burst's afterglow faded, the Caltech team discovered a faint galaxy adjacent to it in the sky, in infrared images obtained with the W. M. Keck Observatory's 10-meter Keck-I telescope at Mauna Kea.

This is almost certainly the galaxy in which the burst originated. The galaxy is about as faint as an ordinary 100-watt lightbulb would be if seen from a distance of half a million miles, about twice the distance to the moon.

Subsequently, following a proposal by the Caltech team and others, the Hubble Space Telescope obtained visible-light images of this galaxy and the burst's afterglow. The analysis of these images by the Caltech team indicates that the galaxy is not unusual in its properties, compared to other normal galaxies at comparable distances from Earth.

A detailed follow-up study of the burst's afterglow by the Caltech team revealed a change in its brightness that could be interpreted as a sign of a jet of energy, moving close to the speed of light, and pointing nearly toward Earth.

"This was the first time that such behavior was seen in a gamma-ray burst," emphasized Kulkarni, "and it may help explain in part its enormous apparent brightness."

Scientists are still debating whether such a powerful beaming of energy occurs in gamma-ray bursts.

The team's findings appear in the April 1 issue of the scientific journal Nature, and in a forthcoming issue of the Astrophysical Journal Letters.

In addition to Kulkarni, Djorgovski, Odewahn, Sari, Bloom, and Gal, the Caltech team also includes Professors Fiona Harrison and Gerry Neugebauer, Drs. Chris Koresko and Lee Armus, and several others.

 

Written by Robert Tindol