Two ultra-high-energy neutrinos captured by the IceCube experiment probably came from outside the Galaxy, according to an analysis posted by the collaboration today.
“We’re pretty excited about it. They’re the highest energy neutrinos that have ever been seen,” says Thomas Gaisser, an IceCube member at the University of Delaware in Newark. IceCube consists of 86 strings of detectors sunk in a cubic kilometre of ice near the South Pole.
The two neutrinos, which were announced in 2012 but had not at that time been analysed, had energies of more than a petaelectronvolt (PeV), which makes them 100 million times more energetic than the neutrinos emitted by supernovae that have reached Earth. One possibility is that they are produced in the atmosphere through the interactions of high-energy cosmic rays there, but given their high energy, that is unlikely, the collaboration finds. If they are instead astrophysical in origin, then they are probably produced by the same sources that make ultra-high-energy cosmic rays, the origin of which is one of the longest-running mysteries in astrophysics. Two possibilities are the gamma-ray bursts emitted from collapsing stars, or active galactic nuclei, the jets emanating from massive black holes at galactic centres, but it’s unknown which. The collaboration puts the possibility of the particles having an astrophysical origin at a statistical confidence level of 2.8 sigma, falling just short of the 3 sigma needed to constitute hard evidence in physicists’ terminology.
IceCube’s detectors (see image) pick up the light emitted when neutrinos and other particles pass through. The two recently analysed neutrinos were plucked from events collected between 2010 and 2012 after the collaboration filtered out lower-energy data. The team is now re-running the analysis with a lower threshold to see whether they can uncover more events from the same sources and reveal their origin.
For years, astronomers and physicists have collected the flighty neutrino particles emitted in the Sun and from supernovae. But being able to observe distant sources from outside the Galaxy would take the field to a new level, and would be a sign that IceCube is now working like a telescope, says Gaisser. “It opens up a new way of looking at the cosmos,” he says.