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There hasn't been a decent supernova in our part of the universe in living memory but astronomers in the 11th century were a little more fortunate. In 1006 AD, they witnessed what is still thought to be the brightest supernova ever seen on Earth (SN 1006) and just 48 years later saw the birth of the Crab Nebula (SN 1054).
Our knowledge of these events come from numerous written accounts, mainly by Chinese and Arabic astronomers (and of course from the observations we can make today of the resultant nebulae).
Now we can go one better. A team of Japanese scientists has found the first evidence of supernovae in an ice core.
The gamma rays from nearby supernova ought to have a significant impact on our atmosphere, in particular by producing an excess of nitrogen oxide. This ought to have left its mark in the Earth's ice history, so the team went looking for it in Antarctica.
The researchers took an ice core measuring 122 metres from Dome Fuji station, an inland site in Antarctica. At a depth of about 50 metres, corresponding to the 11th century, they found three nitrogen oxide spikes, two of which were 48 years apart and easily identifiable as belonging to SN 1006 and SN 1054. The cause of the third spike is not yet known.
That's impressive result and their ice core was obligingly revealing about other major events in the Earth's past. The team saw a 10 year variation in the background levels of nitrogen oxide, almost certainly caused by the 11-year solar cycle (an effect that has been seen before in ice cores). They also saw a number of sulphate spikes from known volcanic eruptions such as Taupo, New Zealand, in 180 AD and El Chichon, Mexico, in 1260 AD.
The team speculate that the mysterious third spike may have been caused by another supernova, visible only from the southern hemisphere or hidden behind a cloud.
That would make the 11th century a truly bounteous time for supernovae. Of course, statistically, there ought to be a supernova every 50 years or so in a galaxy the size of the Milky Way, which means that the Antarctic ice is due another shower of nitrogen oxide any day now.
