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澳门金沙线上娱乐城登录入口:On ice

澳门金沙线上娱乐城登录入口:On ice

作者:巫为  时间:2019-03-07 08:08:00  人气:

By Robert Matthews THE mysterious case of a supernova that exploded on our cosmic doorstep 700 years ago unseen by medieval astronomers seems to have yielded its first terrestrial fingerprint—in the snows of Antarctica. Last year, astronomers reported the discovery by the jointly run German-US ROSAT X-ray satellite of a glowing supernova remnant just 640 light years away in the constellation Vela (New Scientist, 14 November 1998, p 10). The ROSAT observations suggested that the explosion of the star lit up our skies at the beginning of the 14th century, making it by far the closest supernova in our recent past. Astonishingly, however, researchers have failed to find a single written historical reference to this dramatic event, which for a few days would have outshone everything else in the night sky. Now the first terrestrial evidence for the event has turned up in an Antarctic ice core, and resolved a 20-year-old mystery into the bargain. In 1979, analysis of an ice core extracted by scientists at the South Pole revealed the existence of four “spikes” in the concentration of nitrates in the snow (see Diagram). Dating the spikes revealed that three of them coincided with bright supernova explosions that took place in 1181, 1572 and 1604—all of which were recorded by astronomers.FIG-22041001.jpg The spikes are probably the result of nitrates formed when blast waves of ionising radiation from the three supernova explosions struck the Earth’s atmosphere. But the cause of the fourth spike has remained a mystery. Now Clifford Burgess of McGill University in Montreal and his colleague Kai Zuber of the University of Dortmund say this fourth spike is the telltale sign of the explosion pinpointed by ROSAT. Its depth in the ice core corresponds to a date of 1320, give or take 20 years—strikingly similar to the date roughly estimated from ROSAT observations using theories of how supernova remnants evolve. “This fourth spike corresponds precisely with the time when light—including X-rays and gamma rays—from the recently discovered Vela supernova would have been arriving at the Earth,” says Zuber. He adds that the evidence from the ice core also helps us understand the exact nature of this supernova. When combined with standard supernova theory and ROSAT observations, the evidence points to what astronomers call a type II supernova—the obliteration of a colossal star 15 times as heavy as the Sun. The claims intrigue Bernd Aschenbach of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who first uncovered the remnant from among the ROSAT observations. “The coincidence of the nitrate spikes with historical supernovae is striking,” Aschenbach says. “The age is surprisingly close to our early estimate, which had relatively large uncertainties left. This much more precise date is valuable in searching for eyewitness data,