MYSTERIOUS bursts of gamma-rays that astronomers have detected in the sky in recent years may be caused by comets passing through magnetic fields of neutron stars, according to researchers in the Soviet Union.
Over the past 20 years, astronomers have observed several hundred short bursts of gamma-rays. They come from different directions, none of which corresponds to a known astronomical object.
The gamma-rays have such high energies that they must be produced in a very extreme environment. Many astronomers believe that the intense gravitational field around a neutron star must be involved. Neutron stars are very compact stars, no more than 30 kilometres across, in which gravity has crushed together protons and electrons to form neutrons.
One possibility is that the nuclei of comets are hitting neutron stars. But theorists have shown that such collisions would be too rare to account for the number of bursts detected.
Now Roald Sagdeev, project scientist of the Soviet Vega mission to Halley's Comet in 1986, has re-examined the question (Nature, vol 344, p 313). The Vega probes and Giotto, their counterpart from the European Space Agency, found that the nucleus of Halley's Comet was much darker than expected. Previously, astronomers had estimated the mass of comets from their brightness, with the assumption that their surfaces reflect light fairly well.
The results from Halley's Comet show that comets must be about 60 times as massive as previously thought. This means that faint comets - which are much more numerous than bright comets - should be sufficiently massive to produce bursts of gamma-rays.
According to Sagdeev, a comet does not have to hit a neutron star in order to produce gamma-rays. As the comet nucleus passes through the star's magnetic field, intense electric currents are induced in the comet and it vaporises. The ionised matter from the comet then short-circuits the magnetic field, and so produces a burst of gamma-rays. Many more comets will pass through a neutron star's magnetosphere than will hit its surface, so this idea also helps to increase the number of bursts predicted.
Sagdeev also believes that most of the gamma-ray bursts are caused by comet nuclei in interstellar space that just happen to pass unrelated neutron stars. However, astronomers have found two sources that have produced more than one burst. Sagdeev suggests that here, the comets are coming from a cloud around the star, similar to the Oort Cloud that surrounds the Sun. If this is correct, he says, then astronomers studying gamma-ray bursts can use the passage of comets as 'probes' to help us understand the magnetospheres of neutron stars.
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