US Spacecraft to Probe Origin of Gamma Rays（在线收听） |
|
By David McAlary The U.S. space agency, NASA, launched a satellite Saturday that scientists hope will help them locate the sources of mysterious gamma ray explosions, the brightest most energetic bursts in the universe. This is a nearly 40-year-old puzzle. If your eyes could see gamma rays, you would see frequent flashes in different parts of the sky, as if camera bulbs were popping. NASA astronomer Anne Kinney says the fleeting bursts are unpredictable, never occurring in the same place twice. "When we look at the sky with gamma ray glasses, we see 10 second messages from the distant universe,which we call camera bulbs " she said Anne Kinney. "We never know which direction these messages are going to come from, but we know they will be brief - more like postcards than letters." Astronomers say nothing is more powerful than a gamma ray burst. Just one emits more than 100 billion times the energy that the Sun does in a year. "How can you produce so much energy in such a short period of time?" That question is asked by NASA gamma ray expert Neil Gehrels. He says no one knows what causes the flashes, but one view is that they might signal the formation of black holes, pockets of collapsed stars so dense, with such powerful gravity, that not even light can escape. "The gravitational energy of the collapse is a huge amount that could power a gamma ray burst," said Neil Gehrels. "So we think that perhaps bursts are the birth cries of black holes and we're seeing these throughout the universe." Gamma ray bursts were first detected in 1967 by U.S. military satellites monitoring Soviet compliance with the Nuclear Test Ban Treaty. A nuclear blast emits gamma radiation. No treaty violations were detected, but satellite data released in the early 1970s revealed that mysterious flashes were occurring outside our solar system. In the 1990s, spacecraft such as NASA's orbiting Compton Gamma Ray Observatory and the Italian BeppoSax satellite revealed that the bursts occur in all directions and come from distances up to billions of light years away, far beyond our galaxy. The fleeting flashes fade quickly, impeding the quest for more knowledge about them, especially since the detection process is cumbersome. Typically after BeppoSax detects a burst, the information is processed and ground controllers re-command the satellite to observe the portion of the sky where the flash originated. Ground telescopes are alerted to observe in x-ray and optical wavelengths. This entire process can take up to one day. The telescope just launched, called SWIFT, is designed to react instantly. SWIFT mission operations director John Nousek of Pennsylvania State University says the telescope's detectors can sense a gamma ray burst as it begins, causing the spacecraft to spin around rapidly to focus on the event in progress and its afterglow in all the necessary wavelengths. "SWIFT has this tremendous advantage, and it in fact, that was the whole concept of SWIFT of building these all in together, so the amount of latency we have is basically about 20 seconds between when the gamma ray burst occurs and one the satellite finishes analyzing its data set," said John Nousek. "From a scientist's point of view, this is exactly what you are looking for to make progress in a new field." SWIFT's speedy reaction to gamma ray bursts means that the network of ground telescopes can be alerted immediately to provide backup observations. "This responsiveness, that can be useful in a great number of fields of astrophysics," he said. "So that, I like to tell my mission operations people, 'Be ready for that phone call,' because I suspect that phone call may be more important than the whole rest of the mission." As for gamma ray detections, Mr. Nousek says SWIFT will observe 500 to 1,000 events during its initial two-year mission. David McAlary , VOA NEWS, Washington.
|
| 原文地址：http://www.tingroom.com/voastandard/2004/8/3403.html |