In the spring of 1006, stargazers around the globe enjoyed what is thought to be the brightest supernova in recorded history, as observed from earth. Just 48 years later the drama in the heavens resumed as the Crab Nebula was born from the explosive death of another star slightly closer to home. Our knowledge of these events is based on the accounts of Chinese and Arab astronomers along with modern day observations of the supernovae remnants.
Now, a team of scientists based in Japan has discovered that a trace of these explosions has been locked away here on Earth — in the ices of Antarctica.
Yuko Motizuki at the RIKEN Nishina Center for Accelerator-based Science in Wako and her colleagues analysed an Antarctic ice core and identified spikes in the concentration of nitrate ions (NO3–) corresponding to the supernovae of the 11th Century (arXiv 0902.3446).
When intense gamma ray bursts from supernovae in our galaxy interact with Earth’s atmosphere they cause an increase in the production of nitrate ions in the stratosphere. Thanks to atmospheric circulation, some of these ions make it into the Antarctic ice.
Electric swingers
Ice cores are known to be a rich source of information regarding past climates but using them to learn about astronomical phenomena has not moved beyond academic discussions until now.
Motizuki and his colleagues knew that nitrogen oxide transported through the troposphere and lower latitudes tends to precipitate in the coastal region. Therefore, to avoid a distortion in the record, the researchers studied a portion of an Antarctic ice core drilled inland in 2001 at the Fuji dome — the second highest summit of the Antarctic ice sheet.
Another potential source of distortion are the high energy protons originating from so-called “solar proton events” (SPEs). To mitigate against this effect, the researchers calculated the periodicity of these events before choosing which period to analyse. Fortunately, the period of interesting supernova activity in the 11th century coincided with a particularly quiet time for SPEs.
One final measure was to confirm that the spikes were not accompanied by any sudden changes in the water — heavy water (deuterium) ratio, which could indicate sudden violent climate changes.
Closer to home
The depth-to-age relationship of ice was determined by using past volcanic eruption signals as absolute time markers. The section of ice core under analysis — covering a 200 year period — was sandwiched between layers with high sulphate concentrations. These correspond to known volcanic eruptions like El Chichon, Mexico in AD1260.
As well as the spikes, the researchers found a modulation in the background trend of nitrogen oxide levels with a period of 10 years. They suggest that this could represent the solar cycle. Interestingly this periodicity varies from previous nitrogen oxide ice core profiles and theoretical models which report that solar variations occur on an 11-year cycle.
“Our current understanding of solar cycles is totally observation driven. This research significantly increases our understanding of the cycle by going into remote pasts when other solar indices were not available,” said Mausumi Dikpati, a solar magnetic field researcher at the National Center for Atmospheric Research in Colorado.
“I was interested because it is an orthogonal approach to looking at a ‘classical’ astronomical problem,” said Ian Smail a computational cosmologist at Durham University in the UK.
Yuko Motizuki declined to comment as the paper has now been passed to a journal for publication.
