THE mysterious case of a supernova that exploded on our cosmic doorstep 700
years ago unseen by medieval astronomers seems to have yielded its first
terrestrial fingerprint鈥攊n the snows of Antarctica.
Last year, astronomers reported the discovery by the jointly run German-US
ROSAT X-ray satellite of a glowing supernova remnant just 640 light years away in the constellation Vela
(快猫短视频, 14 November 1998, p 10). The
ROSAT observations suggested that the explosion of the star lit up our skies at
the beginning of the 14th century, making it by far the closest supernova in our
recent past.
Astonishingly, however, researchers have failed to find a single written
historical reference to this dramatic event, which for a few days would have
outshone everything else in the night sky. Now the first terrestrial evidence
for the event has turned up in an Antarctic ice core, and resolved a 20-year-old
mystery into the bargain.
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In 1979, analysis of an ice core extracted by scientists at the South Pole
revealed the existence of four 鈥渟pikes鈥� in the concentration of nitrates in the snow
(see Diagram).
Dating the spikes revealed that three of them coincided with
bright supernova explosions that took place in 1181, 1572 and 1604鈥攁ll of
which were recorded by astronomers.FIG-22041001.jpg
The spikes are probably the result of nitrates formed when blast waves of
ionising radiation from the three supernova explosions struck the Earth鈥檚
atmosphere. But the cause of the fourth spike has remained a mystery.
Now Clifford Burgess of McGill University in Montreal and his colleague Kai
Zuber of the University of Dortmund say this fourth spike is the telltale sign
of the explosion pinpointed by ROSAT. Its depth in the ice core corresponds to a
date of 1320, give or take 20 years鈥攕trikingly similar to the date roughly
estimated from ROSAT observations using theories of how supernova remnants
evolve.
鈥淭his fourth spike corresponds precisely with the time when
light鈥攊ncluding X-rays and gamma rays鈥攆rom the recently discovered
Vela supernova would have been arriving at the Earth,鈥� says Zuber.
He adds that the evidence from the ice core also helps us understand the
exact nature of this supernova. When combined with standard supernova theory and
ROSAT observations, the evidence points to what astronomers call a type II
supernova鈥攖he obliteration of a colossal star 15 times as heavy as the
Sun.
The claims intrigue Bernd Aschenbach of the Max Planck Institute for
Extraterrestrial Physics in Garching, Germany, who first uncovered the remnant
from among the ROSAT observations. 鈥淭he coincidence of the nitrate spikes with
historical supernovae is striking,鈥� Aschenbach says. 鈥淭he age is surprisingly
close to our early estimate, which had relatively large uncertainties left. This
much more precise date is valuable in searching for eyewitness data, which so
far have not yet been found.鈥�