Recently Discovered Cosmic Blast May Be Biggest Since Big Bang

By MALCOLM W. BROWNE

Astronomers have detected a titanic explosion in the outer reaches of the

cosmos -- one so violent and bright that for about 40 seconds it appeared to

outshine all the rest of the universe. Except for the Big Bang that apparently

created the universe, no other cosmic explosion of such magnitude has ever

been measured.

The observations that led to this estimate caught theorists completely off

guard, Dr. John N. Bahcall of the Institute for Advanced Study at Princeton,

N.J., said Wednesday. Either new observations of similar explosions will lower

the estimated energy output, or theorists will be forced to seek some entirely

new explanation for the stupendous outpouring of energy.

"I'm a very troubled theorist," said Dr. Stanford E. Woosley of the

University of California at Santa Cruz. "We're really struggling to find a

theoretical basis for this."

The results of an investigation of the explosion by many scientific

institutions in the United States, Europe and Asia were announced at a news

conference held Wednesday in Washington by the National Aeronautics and Space

Administration and are being published in papers Thursday by the journal

Nature.

The 12-billion-year-old event that brought on these frantic international

studies was detected nearly five months ago. On Dec. 14, at 6:34 p.m., Eastern

standard time, an exquisitely sensitive orbiting gamma-ray observatory called

BeppoSAX, which was built by an Italian-Dutch collaboration of astronomers and

launched two years ago, signaled to its operators that it had recorded

something interesting. For about 40 seconds, the satellite measured a sharp

pulse of gamma rays. More important, it pinpointed the position of the rays'

source in the sky.

The Dec. 14 pulse was also detected by the United States' Compton Gamma Ray

Observatory satellite. As measured by the American satellite, the burst's

gamma-ray brightness appeared fairly typical of gamma-ray bursts in general.

But what astronomers did not realize at that point was that the event had

occurred almost incredibly far away, and must therefore have been immensely

powerful to look so bright from Earth.

Thousands of gamma-ray "bursters," as such events are called, have been

detected since the 1960s, but they remain mysterious, because each one

disappears in seconds or minutes and has not been known to recur for further

study. Except for the Dec. 14 burster, none of their distances from Earth have

ever been determined.

But this burster, designated GRB971214, gave astronomers some of the clues

they had long sought.

Right after the gamma-ray burst was detected, dozens of scientific

institutions in many parts of the world raced to look for any traces it might

have left. Gamma rays are the most energetic of all forms of electromagnetic

radiation, but as an object emitting them cools, it radiates waves of lower

energy: X-rays, ultraviolet radiation, visible light, infrared rays and, at

the lowest range of the energy spectrum, radio.

Since BeppoSAX was launched in 1996, its unmatched ability to detect the

exact location of a fleeting gamma-ray burst has made it possible to look for

the lingering afterglow of a burst, which can take the form of X-rays, visible

light and other types of radiation.

Twelve hours after the December burst occurred, Dr. John R. Thorstensen of

Dartmouth College, using a 94-inch-diameter telescope at the Kitt Peak

Observatory in Arizona, found a visible afterglow.

This visible light persisted for about two weeks, and then something even

more interesting turned up at the same spot in the sky: scientists found a

faint galaxy.

Astronomers realized that this galaxy was probably the host of the gamma-ray

explosion and that its distance from Earth must therefore be the same as that

of the burster. Many large telescopes were brought to bear on the galaxy, and

the Keck II in Hawaii, one of the two largest telescopes ever built, hit pay

dirt. A team of American, Italian and Indian astronomers led by Dr. Shrinivas

R. Kulkarni and S. George Djorgovski, both of the California Institute of

Technology, managed to measure the distance to the burster.

Kulkarni said Wednesday that the distant galaxy that spawned the burster

appeared to be creating a myriad of new stars, and that the burst might in

some way be associated with this process.

Three reports by separate collaborations studying GRB971214 are being

published Thursday in Nature. In one, Kulkarni's team described how it had

used the giant Keck instrument to gather enough light from the burster's host

galaxy (dubbed "K") to analyze its spectrum. This was no easy task; the

apparent brightness of the galaxy is only about the same as that of a 100-watt

light bulb viewed from one million miles way.

The astronomers found that lines marking the presence of hydrogen in the

spectrum of light coming from the galaxy had been shifted by a huge amount

from their usual spectral positions toward the red end of the spectrum.

The Keck measurement revealed that Galaxy K is receding from Earth at

enormous speed. This speed, as Edwin Hubble showed, can be used to calculate

the distance of any receding celestial object; the greater its speed of

recession from Earth, the greater its distance.

The "red shift" of light from the galaxy that apparently spawned the gamma-

ray burster was measured as 3.418. From that, the Caltech group calculated

that the explosion occurred about 12 billion years ago, when the universe was

about 15 percent of its present age. This showed that the distance to the

galaxy from Earth was a staggering 12 billion light-years.

From this distance measurement, combined with the measurement of the gamma-

ray intensity observed by the Compton Gamma Ray Observatory, it became

possible to calculate that the energy released by the blast was about 3 times

10 to the 53rd power ergs: an amount several hundred times greater than the

energy released by the explosion of a supernova. Supernova explosions,

produced by certain types of dying stars, are the most violent blasts

astronomers knew until now. Put another way, the energy released during the

40-second gamma-ray burst on Dec. 14 was about equal to the amount of energy

Earth's entire galaxy radiates over a period of several centuries.

(One erg is roughly the amount of energy released when a mosquito flies into

a wall. At the other end of the scale, the energy of the Big Bang has been

estimated at 10 to the 76th power ergs, although the energy density within the

Big Bang is supposed to have been infinite.)

Astrophysicists cautioned that in one unlikely situation, their estimate of

the punch packed by GRB971214 might be too high. If the object released all or

most of its energy along a narrow beam aimed right at Earth, then estimates of

the total energy, released in all directions, would be too high.

Even so, said Dr. Jules P. Halpern of Columbia University, leader of one of

the teams studying the burster, its energy must be enormous.

What could account for the monster explosion of Dec. 14?

"Most of the theoretical models proposed to explain these bursts," said

Kulkarni, of Caltech, "cannot explain this much energy. However, there are

recent models involving rotating black holes, which can work.

"On the other hand," he said, "this is such an extreme phenomenon that it is

possible that we are dealing with something completely unanticipated and even

more exotic."

One possibility favored by some theorists is that a superdense neutron star

might have been ripped apart as it spiraled in toward a neighboring black

hole, and that the disk of debris it formed around the black hole created a

gamma-ray burst just before disappearing into the black hole.

Another member of the Caltech team, Dr. Alexei V. Filippenko, an

astrophysicist at the University of California at Berkeley, suggested that the

burst could have been caused by the violent merger of two black holes.

A question that might one day affect the survival of life on Earth is the

distribution of potential gamma-ray bursters throughout the universe. If most

of them are at enormous distances from Earth, they will pose no significant

threat. But indications are that they are randomly distributed, and that some

must therefore be relatively close to Earth.

The December blast was so big that, in some theoretical predictions, it

might have destroyed the human race had it occurred within a few thousand

light-years of Earth.

In the February issue of Sky & Telescope magazine, two gamma-ray specialists

at NASA's Goddard Space Flight Center, Dr. Peter J.T. Leonard and Dr. Jerry T.

Bonnell, described gamma-ray bursters as potential killers.

They quoted two astrophysicists at the Israel Institute of Technology, Dr.

Nir Shaviv and Dr. Arnon Dar, as having calculated that if a burster occurred

within 3,000 light-years of Earth, the flood of high-energy gamma rays

bombarding Earth's atmosphere would spawn a cascade of cosmic-ray particles,

including vast numbers of deeply penetrating muons. The muon radiation at sea

level would be about 100 times the lethal human dose, the Israelis calculated,

and would kill organisms even hundreds of yards under water or under ground.

Moreover, such a blast would create long-lived radioactive isotopes that

would poison the terrestrial environment for many millions of years.

More than 2,000 bursts have been detected since 1991. Within Earth's galaxy,

five pairs of neutron stars that might merge to become gamma-ray bursters are

already known, and there may be hundreds more, Leonard and Bonnell said,

although the soonest that any one of the five candidates could merge and

explode is 220 million years from now.

The Israeli astrophysicists estimate that about once every hundred million

years one of these cosmic bombs might go off near enough to Earth to cause

harm. They speculate that gamma-ray bursters may have been one of a variety of

causes of mass extinctions of species that have occurred during Earth's

history.

"Gamma-ray bursters are definitely a potential threat to life on earth,"

Filippenko said. "But happily, we don't see a statistical likelihood of one

going off near us for many millions of years.

"I don't even think GRB971214 killed off any civilization in its host

galaxy. The galaxy was too young to have developed a civilization, although

there might have been some primitive form of life that came to grief."

Other Places of Interest on The Web NASA Structure and Evolution of the Universe.

Nature Journal.

Thursday, May 7, 1998