Gamma-ray bursts linked with stellar explosions

Artist's conception of a gamma-ray burst from a massive star

September 29, 1999
Web posted at: 3:55 p.m. EDT (1955 GMT)

In this story: Spectral findings point to supernova Gamma rays discovered in 1967 RELATED STORIES, SITES

(CNN) -- Gamma-ray bursts, the most powerful explosions in the universe, may come from the formation of black holes during the fiery deaths of extremely bright, rare stars called supernovae, astronomers said Wednesday.

The finding was sparked by a short-lived burst on March 26, 1998, spotted by the Italian-Dutch satellite BeppoSax, which relayed the event's position.

Caltech astronomers using the Keck Telescope in Hawaii analyzed the burst's afterglow, called GRB 980326, and said they were surprised to get spectral data indicating it had rebrightened dramatically, obscuring the galaxy itself.

They had expected that the burst's afterglow would quickly be replaced by the general brightness of the surrounding galaxy.

The observation left them more convinced that similar brief, but brilliant cosmic flashes are generated by the collapse of rare, massive stars called supernovae.

The brightening the astronomers saw was the underlying supernova created when the massive star exploded, said Caltech astronomer Shrinivas Kulkarni.

"Galaxies don't just disappear," Kulkarni said. "This is something quite new."

The findings were published in the latest issue of the journal Nature.

Spectral findings point to supernova

A fading source observed over the span of 9 months with the W. M. Keck 10-meter Telescope in Mauna Kea, Hawaii (Courtesy Caltech)

The rebrightening and the spectrum changes can be explained by presence of a supernova, said S. George Djorgovski of Caltech in Pasadena, California.

The intensity of the rebrightening matched the peak brightness of a supernova seen in a distant galaxy, and its red spectrum also had the right color, he said.

"The spectrum of the source right after the burst was blue, which is common," he said. "But after a month it was very red, which was unexpected.

"That alone suggested that we were looking at some different phenomenon happening at the same location, but with a time delay of a few weeks."

According to the astronomers' theory, a black hole quickly forms in the center of a dying massive star whose core cannot support itself against gravity.

When the star explodes, powerful jets of high-energy particles are ejected into space.

Gamma rays are thousands of times more powerful than the energy emitted by our sun over its entire lifetime.

Kulkarni said gamma ray bursts are seen only when the jets from a supernova's central black hole is pointed directly at Earth.

That makes gamma ray bursts seem like random occurrences at random places in the sky, when in fact they are associated with a particular events.

Gamma rays theories are common

Gamma rays were discovered in 1967, and astronomers observe them once a day on average.

They have higher energy than all other forms of radiation, but that energy never affects Earth's surface because it is absorbed by the atmosphere.

There are more than 150 theories to explain the origins of gamma rays, but the Caltech team's supernova theory has gained widespread support in recent years as their observations of gamma ray explosions pile up.

Earlier this year, the most powerful explosion ever observed -- a deep space eruption detected 9 billion light years from Earth -- was a gamma ray burst that astronomers say probably was associated with a supernova explosion, too.

Two other groups, from the universities of Amsterdam and Chicago, in view of the work presented by the Caltech team, reanalyzed data on some other gamma-ray bursts and found support for the findings of Kulkarni and his colleagues.

The Associated Press contributed to this report.