Every day or so a mysterious burst of gamma rays reaches Earth from far-off space. One burst packs the energy of a thousand suns.
Ever since their discovery 24 years ago, gamma ray bursts have baffled astronomers. These brief high-energy flashes erupt for a few seconds, then fade away, never appearing in exactly the same place in the sky twice. Until last spring many astronomers believed that the gamma ray bursts came from somewhere within or near our own Milky Way galaxy, although no one knew for sure what caused them. But new data from NASA’s Compton Gamma Ray Observatory satellite, launched in 1991, have made a local origin seem increasingly unlikely. Gamma ray bursts, whatever they are, probably lie billions of light-years beyond the Milky Way galaxy. They may be the most energetic phenomena in the universe.
For more than two years now the Compton Observatory has been detecting bursts at a rate of nearly one a day, for a total of over 600. They appear uniformly across the sky, surrounding Earth in a spherical shell of fireworks. This symmetrical distribution is what makes astronomers suspect that the bursts occur far outside the galaxy, perhaps at the very edge of the observable universe. If the bursts were inside the Milky Way, their distribution, seen from Earth’s vantage point on one side of the galaxy, would look skewed--there would be more of them toward the galaxy’s center.
This perfectly symmetrical distribution is telling us these things have nothing to do with our galaxy, says Bruce Margon, an astronomer at the University of Washington in Seattle. Instead they are in very distant galaxies, galaxies so far away that they fill the entire celestial sphere uniformly. In that case we’re dealing with events whose intrinsic energy is, dare I say, astronomical.
Indeed, Stan Woosley, an astronomer at the University of California at Santa Cruz, has calculated just how energetic the bursts must be in order to look as bright as they do from such a great distance. In ten seconds, he says, a gamma ray burst may release a thousand times as much energy as the sun will generate in its entire 10-billion-year lifetime.
What process could possibly release such a flood of energy? Woosley has a few ideas. All of them involve dense collapsed stars called neutron stars, or even denser black holes. The gravitational field of a neutron star, for example, is so powerful that objects falling onto it release incredible amounts of energy. Drop a pound of sand onto a neutron star and the sand would explode with the force of a million tons of TNT. Well, let’s not drop a pound of sand, says Woosley. Let’s drop a neutron star on a neutron star. Such a collision would release enough energy for a gamma ray burst. And since astronomers know of several pairs of orbiting neutron stars in our galaxy alone, such crack-ups may not be rare in the universe.
Another possibility, says Woosley, is that gamma rays may burst out when the core of a giant star caves in under its own weight to become a black hole. Over a period of several minutes the rest of the star is consumed by the black hole, he says. Some of the material could be heated to such high temperatures as it falls in that it might emit a burst of gamma rays.
Finally, some astronomers believe the reason the bursts are so bright is that the gamma rays are emitted in tight beams--perhaps because they are focused by a black hole’s powerful magnetic field--rather than in all directions. If the gamma rays were traveling along parallel paths in a narrow beam, fewer of them would collide with and annihilate one another, and more of them would make it to Earth. One of the fascinating implications is that there might be a whole lot of bursts we don’t see, says Woosley. Most of them are pointed this way and that, and we just happen to lie in the beams of the ones we see. It’s incredible enough to see one of these every day. But there might be one every hour, maybe even one every minute.
Ultimately astronomers would like to identify a gamma ray burst as some recognizable celestial body, something they have so far failed to do. The difficulty is that the bursts flare up and fade away so suddenly that astronomers don’t have time to train other telescopes on them once they’ve been spotted by the Compton Observatory. There’s a fundamental problem in that you don’t see anything when the burst isn’t active, says Woosley. If we could find just one source that was still glowing after emitting a gamma ray burst, it would crack this problem wide open.