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More Violence Overhead

Bursts of gamma rays have been a mystery for 30 years; Now, with new satellites, we have some clues

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Violence fascinates us. We watch football and stock car racing. We can't seem to get enough of "action" movies, with their car chases and stuntmen crashing through windows and people being killed up close and personal. Sporting names like Jaws & Claws, a new breed of animal shows, in which predators are depicted as crazed killers, has reached television. Even in real life, we find it impossible to drive by a serious accident without slowing down to look.

Astronomers see violence on a, well, astronomical scale. We've known for a generation just how violent a place the Universe is. Before that, we had known about novas, stars that blow off their outer shells, and supernovas, stars that simply blow up, leaving only a core behind. Those cores are neutron stars which, when they are oriented just so, are also the pulsars that so excited the world when they were discovered in 1967.

We know that galaxies really do collide, setting off pyrotechnic displays of new, hot stars that form as the gases in those galaxies are compressed. Now, after a generation of theorizing, we think that we have solved the mystery of quasars, faint fuzzy "stars" that emit prodigious quantities of energy. They probably are black holes at the centers of galaxies, pulling in the interstellar gas as well as the stars around them, ripping them apart in the process. As stellar material is accelerated to relativistic velocities by the black hole's gravitational field, it gives off radiation that is more and more energetic, as if the very life were being pulled out of it. That radiation is the "scream" of a star dying a violent death.

Now we're on the brink of solving another 30-year-old mystery, one that also involves the release of prodigious amounts of energy. Military satellites were the first to see what are called "gamma ray bursts" (GRBs), seconds-long eruptions of photons even more energetic than x rays. Gamma rays do not produce sharp images, however, so it was difficult to pinpoint where a burst had occurred. And because we had no clue as to whether the bursts were happening in our own galaxy or in other, distant galaxies, we did not appreciate just how powerful the bursts really are.

The tempo picked up in 1991 when the Compton Gamma Ray Observatory was launched. It carried an apparatus specifically designed to detect and locate GRBs. It saw about one a day, and it saw them in all directions: they were not concentrated along the Milky Way, our home galaxy. This suggested they were happening outside our galaxy, very far away. In 1996 an Italian-Dutch satellite called BeppoSAX carried a GRB monitor and x-ray cameras and telescopes that would detect the gamma rays themselves, and locate the bursts by means of the x rays that were suspected to stream from them in an afterglow. Using BeppoSAX, astronomers located their first GRB on January 11 of last year. On February 28, they detected a burst for which, for the first time, an optical counterpart was found: a glow of visible light. After another burst on May 8, one of the world's most powerful telescopes, at the Keck Observatory in Hawaii, obtained a spectrum that revealed the presence of a large cloud between the light source and Earth, about seven billion light-years away. The burst we saw on May 8 had actually happened at least seven billion years earlier, long before Earth existed. More important, for the burst to be visible so far away, the amount of energy released had to be extraordinary. It is difficult to wrap our minds around such numbers, but my favorite comparison goes thusly: a gamma ray burst puts out more energy in 15 seconds than the Sun will release in its entire ten-billion-year lifetime.

So we have the same challenge we had with the quasars: show us the mechanism. The leading candidate so far seems to be the violent merging of neutron stars. These are supernova remnants in which the remaining matter has been crushed together so that the protons and electrons of ordinary matter have been pushed into each other to form neutrons. Most matter is almost entirely empty space, but not this stuff: a teaspoonful would weigh several tons. So when neutron stars collide, the kinetic energy is off the scale.

Whatever causes a burst, what we see is the resulting fireball. We cannot see the event itself, because in the fireball's earliest stages, the plasma inside is so dense that no electromagnetic radiation can escape. Only when the fireball has reached out 60 million miles or so, moving at close to the speed of light, do the gamma rays and some x rays escape and we detect a GRB. When it has reached about 60 billion miles, it is thought to collide with interstellar gas, an event that emits more x rays, along with visible-light and radio waves. It was a spectrum of those visible wavelengths that gave us a minimum distance to the burst of May 8. Radio astronomers reported in September that the fireball from that burst was then about a tenth of a light-year across; that represents 170 times the distance from the Sun to Pluto.

Astronomers who had thought the GRBs were close by, in our own galaxy, have been disappointed, but the Milky Way still holds a gamma ray mystery of its own for them to unravel. In November a team of scientists from three universities an-nounced that a huge cloud of gamma rays forms a halo around our galaxy. Once again, we see the gamma rays (courtesy of the Compton spacecraft), but we don't know what is generating them. They don't appear to be coming from discrete sources. Three possibilities are being looked at: high-energy cosmic rays colliding with photons of lower energy light, such as visible or infrared; rapidly spinning neutron stars or pulsars, some of which emit most of their energy as gamma rays; and, even more intriguing, dark matter (Smithsonian, June 1993), which might somehow be producing the gamma rays. Weakly interacting massive particles (WIMPS)--theoretical particles that may constitute dark matter--would be expected not to interact with light. But if they exist and form a halo around our galaxy, it could be that when they collide with each other, they generate either gamma rays directly or paired particles of matter and antimatter that then annihilate in a burst of gamma rays.

Whether you look seven billion light-years away, or just outside our own galaxy, you find gamma rays being produced by what can only be violent processes. The amounts of energy involved dwarf anything we can imagine. Only now are we beginning to get a handle on what's going on. The next step, as always, is more research. In a few years new satellites, with names like HETE (High Energy Transient Explorer), will fly to give us more and better data. Sooner or later, we'll stamp "solved" on the GRB folder. Then, and remember you read it here first, new mysteries will arise.

In the meantime, when you look up on a clear winter night, don't be fooled by the apparent peace and orderliness. It's dangerous out there.


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