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Scientists have long wanted to observe the moment when a star collapses into a black hole. And indeed, a phenomenon that seems to correspond to that moment has emerged: a celestial event called a gamma-ray burst.
During the 1960s, at the height of the Cold War, the United States launched artificial satellites to detect gamma rays discharged during nuclear-weapons tests, in order to monitor the nuclear activities of the Soviet Union. Fortunately, there were no gamma rays coming from Earth, but instead scientists discovered a mysterious phenomenon in which from time to time cosmic gamma rays rushed like tsunamis. This phenomenon was named a gamma-ray burst, but the timing and location of these bursts remained unpredictable. In addition, they were very brief, brightening for just a few seconds or minutes, and then disappearing. Even when we were fortunate enough to able to determine where the bursts would likely occur, when we searched those areas with telescopes later we could not find any suspicious-looking celestial bodies.
Finally, in 1997, an Italian satellite managed to pinpoint the location of a gamma-ray burst just a few hours after it occurred. Using optical telescopes on the ground, astronomers were then able to discover the afterglow of an explosion that it faded far more slowly than the gamma rays. After the afterglow had disappeared, the location was examined in visible light. What turned up was an extremely distant galaxy - a galaxy that existed when the universe was still young. Consequently we learned that gamma-ray bursts are likely big explosions that happen at the farthest corners of the universe. The fact that we can detect gamma rays that are discharged so far away leaves no doubt that these explosions release gigantic amounts of energy.
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To observe gamma-ray bursts as early as possible, Japan's Institute of Physical and Chemical Research launched the satellite HETTY II, in cooperation with France and the United States. This satellite has been keeping a close watch on the occurrence of gamma-ray bursts around the universe. Once it detects a burst, it determines its location with a margin of error of around one-third of the full moon. The location is immediately transmitted to the ground station, and broadcast all over the world via the Internet. This causes a large number of stand-by automatic telescopes to start looking for an afterglow.
Dr. Kenichi Torii of the Institute of Physical and Chemical Research operates three such telescopes, each with an aperture of 25 cm. HETTY II has detected more than 100 gamma-ray bursts so far, with one of the largest detected on the March 29, 2003. Dr. Torii was the first researcher in the world to detect the afterglow of this giant burst. Here you can see the emitted light gradually disappear over approximately two nights. Further pursuit of this afterglow with larger telescopes revealed that its spectrum gradually changed about one week after the burst. It changed from synchrotron radiation emitted by high-energy particles during the explosion, to a spectrum that is unique to a supernova explosion. This has come very close to proving the occurrence of the following chain of events: a massive star explodes in a supernova, causing the star's core to collapse and form a black hole, at that moment releasing bursts of gamma rays.
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