Is it a deadly duet or a heavyweight turning itself inside out? These are two of the leading theories that have emerged after three decades of study of random violence at the edge of the observable universe.
Gamma-ray bursts were discovered in the late 1960s by satellites monitoring the nuclear test ban treaty. Scientists since have identified two main causes of bursts, giving them two puzzles to solve.
One cause of this phenomenon is neutron stars, in or near our galaxy, with magnetic fields strong enough to brake the star's rotation. The other is true gamma ray flashes which, in the last three years, scientists have been able to pin down as coming from the edge of the known universe.
More than 150 possible causes were listed at the First Huntsville Gamma Ray Burst Symposium, said Dr. Stan Woosley of the University of California at Santa Cruz, speaking to the Fifth Symposium, held this week. Most have been thrown out. (Included in this batch was the suggestion, offered up by a tabloid newspaper referring to the famous March 5, 1979 burst, which was actually caused by a magnetar, that the event was produced by advanced civilizations duking it out a la Star Wars.)
Only a few have become serious contenders. Among these are the coalescing neutron star theory and the collapsar theory.
"We shouldn't try to explain everything with one model," Woosley said. "We should try to push the model as far as possible, but not be surprised that the universe has more going on than we had expected." Woosley spoke at the Fifth Biennial Huntsville Gamma Ray Burst Symposium, which concluded Friday.
The coalescing neutron star theory, offered in several camps, sketches a violent end for two partners. Coalescence happens when the gravitation forces between two neutron stars (left after massive stars had blown up as supernovae) gradually shorten their "year" to days, hours -- even seconds. At the same time, their own gravity distorts and ruptures their bodies, and the two collapse into each other and form a black hole.
The collapsar theory is no less exotic or violent. A supernova happens when a star about 10 to 25 times as massive as our sun has burned everything in its core until it is mainly iron and nickel nuclei "ash." Any further fusion would absorb energy instead of producing the energy that props up the overlying layers of gas.
It's like flipping the off switch. The star collapses on itself, then rebounds outward in a catastrophic explosion that shines for intelligent life forms galaxies away.
If the star is too massive, the supernova fails and instead becomes something more powerful, Woosley predicts. The supernova explosion starts, but the shock wave only gets a few hundred kilometers out, slows and collapses under the overlying mass.
Now it gets weird. The star turns itself inside out.
Woosley and his colleagues have conducted computer simulations that show the collapse will generate two jets of superhot matter along the star's rotational axis. It becomes a massive geyser tunneling to the surface of the star. When it emerges, the geyser spews material into space. An intense shock wave wraps around the star and tears it apart as the core collapses even farther. The result is a relativistic fireball - so-called because it's moving near the speed of light - moving outward and a black hole gobbling up anything left behind.
The whole event takes less than two minutes.
Collapsars can involve stars ranging from red supergiants to hot, young helium stars. "Even this simple model can get you into a lot of complexity," Woosley cautioned, and noted that more study and simulations are needed. While that is being done, scientists are preparing new instruments to collect more observations to guide the theorists.
The most unusual is MILAGRO - the Miraculous Gamma-Ray Observatory - at Los Alamos National Laboratory. It looks more like a swimming pool than an astrophysics observatory. In it are instruments to detect light flashes from secondary particles, released when super-high-energy gamma rays and cosmic rays plow into Earth's atmosphere and cause flashes of light in the darkened water.
A more conventional approach will be taken with the High-Energy Transient Explorer (HETE II), scheduled for launch in January 2000. It will look into deep space, away from the sun, and watch for bursts in ultraviolet and X-rays, as well as gamma rays.
Next will come Swift, a recently approved Explorer-class mission. Swift will carry a hard X-ray detector to alert its computer so the spacecraft can aim itself in a few seconds and train an array of telescopes on the burst. It is scheduled for launch in 2003.
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