Recently Discovered Cosmic Blast May Be Biggest Since Big Bang
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Closing In on Big Bang, One Galaxy at a Time (May 5)
Once Upon a Time, There Was a Big Bang Theory (March 8)
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.
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.
Copyright 1998 The New York Times Company