The Gamma-Ray Burst Next Door
by Christopher Wanjek
New evidence suggests that life-threatening explosions are closer than we thought.
I hesitate to spawn a thousand bad sci-fi flicks, but here it goes: Scientists now say that some gamma-ray bursts, the most powerful explosions in the universe, originate in nearby galaxy clusters. If one were to occur nearby, it could wipe out life on Earth.
Fortunately, the chances of mass extinction are slimmer than the Chicago Cubs meeting the Boston Red Sox in the World Series (. . . and the Red Sox winning). But a new analysis of over 1400 archived gamma-ray bursts reveals that about 100 bursts originated within 325 million light-years of Earth, and not billions of light-years away as previously thought. If so, there's no reason why a burst couldn't go off in our galaxy.
Jay Norris of NASA's Goddard Space Flight Center discovered the nearby clustering, representing a new subclass of gamma-ray bursts. He presented these results at the American Astronomical Society's January meeting. The bursts, Norris said, form an oblate distribution toward the Supergalactic Plane, an imaginary plane that slices through several surrounding galaxy clusters.
This is exciting news for astronomers. The bursts may be associated with certain stellar explosions called Type Ib/c supernovae and may, in fact, provide a warning that a supernova event is imminent. The bursts' presence could explain the long-standing mystery of ultrahigh-energy cosmic rays. And the bursts could be sources of detectable gravitational waves.
Frequent and random, powerful and mysterious, gamma-ray bursts fade so quickly that scientists have been unable to determine their source. Most gamma-ray bursts last only a few seconds. The shortest bursts could be black hole or neutron star mergers, while longer bursts are probably massive, albeit theorized, stellar explosions called hypernovae.
In 1999, Norris and his Goddard colleagues found a relationship between the distance to a burst, its luminosity, and its so-called "lag time." In any given burst, the high-energy gamma-ray photons arrive slightly faster than the lower-energy gamma-ray photons. More luminous bursts seem to have shorter lag times. Comparing the measured burst luminosity with the intrinsic luminosity (actual brightness, determined by lag times) yields a distance.
By characterizing gamma-ray bursts in terms of lag time and luminosity, Norris found that most of the 1437 archived burst profiles he studied had high luminosities and originated billions of light-years from Earth, as expected. About 100, however, were of lower luminosity. Norris speculates that this type of burst is created by the collapse of very large stars, perhaps 10 to 50 times as massive as the Sun. The bursts are nearby, occurring about as frequently as an unusual class of supernovae called Type 1b/c.
If some gamma-ray bursts are just big stellar explosions, then they have likely occurred in the Milky Way Galaxy within the past few hundred million years. According to Enrico Cappellaro of the Padova Observatory in Italy, a galaxy like ours is home to a Type 1b/c supernova about once every 500 years. But recent evidence suggests that many or all gamma-ray bursts beam most of their energy in bipolar jets, and the probability that Earth would lie within the beam of any particular burst is about 1%.
Still, these are violent events we're talking about. Steve Thorsett, now at the University of California, Santa Cruz, calculated that a gamma-ray burst originating in the center of our galaxy, about 30,000 light-years away, would tickle Earth's ozone layer causing damage comparable to a large volcanic eruption.
If a burst pops off within 3000 light-years from Earth, then we've got trouble. A 10- second burst would wipe out the ozone layer for several years, and the influx of solar ultraviolet radiation would cause severe skin cancers and kill vegetation, says Thorsett. Gamma rays colliding with atmospheric nitrogen would create dark nitric oxide skies, chilling Earth's surface and acidifying the rain. Interestingly, merely one close burst every 100 million years would be consistent with known periods of mass extinction on our planet.
Are you scared? Well, statistically speaking, we're 50 million years away from the next local burst. Gamma-ray bursts are not all about destruction anyway. Brian McBreen and Lorraine Hanlon of University College Dublin suggest that bursts may foster life. A burst of gamma-rays encountering a planetary nebula creates sticky molten dust, quite possibly paving the way to terrestrial planet formation.
NASA's Swift mission, scheduled for a 2003 launch, could confirm the existence of nearby bursts by detecting the less luminous bursts with longer lags. Swift will also quickly determine the precise burst location. If the bursts are associated with Type 1b/c supernovae, they would appear just before the supernovae, providing scientists with advanced warning to witness an entire supernova event.
Mass extinction aside, that would be quite the movie to see: a gamma-ray burst announcing the beginning of a supernova. But bring the sunblock just in case.
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