When Comet Hyakutake streaked close to Earth last week, a scientific spacecraft made a discovery that has astonished and puzzled astrophysicists. The comet was emitting X-rays in a crescent pattern on its sunward side.
Cosmic radiations as powerful as X-rays are usually produced by cataclysmic forces. They are associated with the extremely hot gases spread by exploding stars or the tremendous accelerations of particles caught in the gravitational whirlpools around dense neutron stars or even denser black holes.
So what were X-rays doing coming from a comet, a relatively benign iceball?
Although some astrophysicists had speculated on the possibility of comet X-rays, no one had ever observed them or expected to detect emissions as strong and rapidly variable as those observed March 27 by Rosat, a German X-ray astronomy satellite.
A team of American and German astrophysicists, led by Dr. Carey M. Lisse of the Goddard Space Flight Center in Greenbelt, Md., reported the discovery yesterday in an announcement issued by NASA.
Computer processing of the Rosat data determined that when the comet was less than 10 million miles from Earth, its closest approach, it was emitting X-ray signals about 100 times stronger than anyone had ever predicted.
Scientists were also surprised by the pronounced increases and decreases in the X-ray brightness from one observation to another, typically over a few hours.
"It's just an enormous surprise," said Dr. Robert Petre, an X-ray astronomer at Goddard. "We really don't know what causes these radiations."
As a start, astrophysicists were considering two possible explanations for the strong X-ray emissions.
One hypothesis is that X-rays from the sun were absorbed by a cloud of gaseous water molecules surrounding the nucleus of the comet. Then they were re-emitted by the water in a process physicists call fluorescence. But if that is the case, some astronomers asked, why did the emissions appear in the shape of a crescent and not a sphere?
A second possibility, scientists said, is that the X-rays were being produced from violent collisions between the comet's atmosphere and the supersonic "wind" of particles and electrified gases known as plasma streaming from the sun.
"They're plausible ideas," Dr. Petre said. "But I'm not sure we know enough yet. We may need to correlate these data with other observations of the comet, particularly the behavior of water in the comet's atmosphere."
Pub Date: 4/05/96