Paths of glory

日期:2017-09-14 01:44:29 作者:牛浣 阅读:

By Marcus Chown GAMMA-ray bursts may be produced by the debris expelled from a supernova in a hyperfast jet, according to a Princeton astronomer. His theory would also explain why some pulsars fly through space much faster than normal stars. Gamma-ray bursters, which exist at the edge of the Universe, are the source of the most powerful blasts of energy known. Astronomers can only guess at what makes them pack that kind of punch, but the discovery last year of X-ray and optical “afterglows” associated with these bursts suggests that they might be the fading fireballs of some kind of stellar explosion. Now Renyue Cen of Princeton University in New Jersey is suggesting that these intense bursts of energy might come from a supernova that is expelling material far faster in one direction than in others. In a paper to appear in Astrophysical Journal Letters Cen speculates that some unknown process sweeps a path free of protons and neutrons, allowing the neutrinos formed within the exploding star to escape. Some of the neutrinos decay into electrons and positrons, forming a jet travelling at about 99.9994 per cent of the speed of light. The electrons would emit light as they interacted with magnetic fields. Because they are moving so fast, the Doppler effect would boost this radiation to gamma-ray frequencies, producing the burst that we observe. Cen’s theory would also explain why some pulsars are moving at up to 500 kilometres per second—tens of times faster than ordinary stars. “A supernova jet propels a pulsar in the opposite direction just like the exhaust of a rocket,” says Cen. “There is good reason to believe that at least one gamma-ray burster is associated with a supernova and I’m not surprised if high-velocity pulsars are associated with supernovae too,” says Andy Fabian of the University of Cambridge. Proof could come if scientists observed a gamma-ray burst and a supernova going off together. But this will not be easy. “Unfortunately, supernovae are extremely faint and hard to spot at the typical distance of gamma-ray bursters,” says Cen. He also suggests that all supernovae may produce superfast jets, but that we only see them as a gamma-ray burst when the jet points our way. Since several supernovae are known to go off every century in our Galaxy, Cen believes that every few hundred million years the Earth could find itself looking straight down the jet of a nearby gamma-ray burst—with extremely serious consequences. “The effect on life on Earth would be catastrophic and might well trigger mass extinctions,