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No. 106: Jul-Aug 1996

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The tale began on March 27, when Comet Hyakutake passed within 15 million kilometers of earth. At this point in its trajectory, it came into the field of view of the X-ray astronomy satellite ROSAT. ROSAT was designed to look at stars whose extremely high temperatures can generate X-rays. It seemed ridiculous to point ROSAT's instruments at a comet composed mainly of ice and dust. How could a comet emit X-rays? When a German-American team of scientists proposed taking a peek at Hyakutake with ROSAT, scientific eyebrowns were raised. What a waste of observing time!

At the most, the team thought they might pick up a smudge of weak X-rays where dust grains flying off Hyakutake collided with dust grains normally present in interplanetary space. The team did get ROSAT to take a look, and what the satellite saw ignited a controversy.

Some 50,000 kilometers in front of the comet was a bright crescent of X-rays, 100 times brighter than the brightest "smudge" the team of scientists had hoped for. This was completely unexpected. All astronomers could do was come up with three rather unconvincing theories: (1) Solar X-rays were absorbed and reemited by the comet (Xray fluorescence); (2) Cometary material emitted X-rays when bathed in the solar wind; and (3) Charged particles were somehow accelerated by a magnetic field compressed by the comet's bow wave in the solar wind. Nobody is particularly happy with any of these theories.

(Hecht, Jeff; "Comet Bids Farewell with Blaze of X-rays," New Scientist, p. 18, April 20, 1996. Glanz, James; "Comet Hyakutake Blazes in X-rays," Science, 272:194, 1996)

Comment. Serendipity has struck again! Imagine all the phenomena we are missing because we know they can't happen!

From Science Frontiers #106, JUL-AUG 1996. � 1996-2000 William R. Corliss