Starquake turned night into day


By Jeff Hecht FOR five minutes on 27 August, the Earth’s upper atmosphere was electrified by a burst of gamma rays from a distant star. The event has refined astronomers’ views of a strange class of stars called magnetars. The source of the gamma rays was a star called SGR1900+14, which has previously emitted similar but smaller bursts. Astronomers now believe that such “soft gamma-ray repeaters” are young neutron stars with intense magnetic fields. According to theory, the motion of the magnetic field through the neutron star’s iron crust heats it to millions of degrees until it cracks apart in the stellar equivalent of an earthquake, releasing gamma rays (“Starquake”, New Scientist, 15 August, p 26). The Earth’s upper atmosphere is ionised by solar radiation during the day, but becomes de-ionised at night. Although SGR1900+14 lies 20 000 light years away, its burst ionised the atmosphere of the Earth’s dark side down to an altitude of 60 kilometres. “It looked just like daytime,” says Umran Inan, an astronomer at Stanford University in California. The gamma rays saturated the detectors of two spacecraft and triggered an automatic shut-off of a third. Kevin Hurley of the University of California at Berkeley estimates that the starquake released as much energy in 5 minutes as the Sun emits in 300 years. Fluctuations in the intensity of the gamma rays fit well with models of the behaviour of magnetars,
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