Bert and Ernie and a new type of astronomy?

In May 2013, scientists presented a preliminary analysis of 28 high-energy events captured by the IceCube Neutrino Observatory, a strange telescope entombed deep in Antarctic ice. Two of these events – dubbed Bert and Ernie – had an energy above 1 PeV. (I wrote about these events in an earlier post.) The other 26 events had an energy in excess of 30 TeV. The initial analysis suggested that these 28 events were likely to be from extraterrestrial sources. A more detailed analysis, published today in the journal Science, suggests that only about 11 of the 28 events are likely to have been caused by atmospheric muons or neutrinos. This means that, at a 4? level of certainty, IceCube has detected high-energy neutrinos from outside the Solar System. A 4? result is not quite at the 5? level that is usually said to constitute a discovery, but it is highly suggestive: there is only one chance in 15000 that all those detections were of purely atmospheric events.

IceCube building

The IceCube Neutrino Observatory consists of dozens of photomultiplier tubes attached to 86 cables, each of which are up to 2.5 km long and buried deep in Antarctic ice. The photomultipliers detect the Cerenkov radiation from fast-moving secondary particles created when neutrinos strike nuclei in the ice. The structure here is just the tip of the observatory! (Credit: IceCube Collaboration)

The exciting thing, I believe, is that the IceCube team now know how and where to look for high-energy neutrinos. They’ll find more astrophysical neutrinos, for sure, and the neutrino sky suddenly looks much more interesting. For many years, the only extraterrestrial neutrinos that astronomers had detected were those from the Sun and a few from SN1987A. IceCube has thus broken new ground.

The IceCube discovery has caused many commentators to hail a new type of astronomy: neutrino astronomy. Well, I don’t think we are quite there yet. The problem is that we don’t know where Bert, Ernie or the other neutrinos originated. To do neutrino astronomy one needs to be able to correlate neutrinos with specific astrophysical objects; the IceCube measurements lacked the angular resolution to do this. But that, too, will come. And new neutrino telescopes, such as the KM3NeT facility that is being constructed in the Mediterranean, will help.

We can’t do neutrino astronomy just yet, but it won’t be long before we’re studying the universe from an entirely new vantage point. And then, for the first time, astronomers will be able to study the distant universe using something other than electromagnetic radiation. IceCube is opening its eyes.