Earlier this year, scientists using a powerful detector at the South Pole discovered Ernie and Bert, two neutrinos with energies over 100 times higher than the protons that circulate in the LHC. Now, the same team has combed through its data to find an additional 26 high-energy events, and they've done a careful analysis to show that these are almost certainly originating from somewhere outside our Solar System.

Neutrinos are incredibly light particles that rarely interact with normal matter; staggering numbers pass through the Earth (and your body) every second. To spot one, you need a very large detector, and IceCube fits the bill. Located in the ice cap at the South Pole, the detector works by capturing the light produced when neutrinos interact with the huge volume of ice present. To do so, holes were drilled up to 2 km into the ice, and strings of photodetectors were lowered into them. All told, they pick up the signals from a cubic kilometer of ice.

The challenge is figuring out which signals come from the out-of-this-world neutrinos. Cosmic rays slam into the atmosphere all the time, and these can produce neutrinos that then enter the ice cap. They can also produce other exotic particles that produce light as they pass through the ice. Muons, for example, only live about 10-6 seconds, but they're moving so fast that time dilation means they live longer from the Earth's frame of reference. As a result, they may travel several kilometers through the ice before decaying.

To handle these cases of background, the authors eliminated any signals that were present in the outermost edges of the detector. Cosmic rays are especially easy to spot given that they tend to produce a spray of particles, many of which will be found at the detectors closest to the surface. You might still get a few neutrinos created above the North Pole and passing through on their way out of Earth, but the authors found that the majority of their signals came from the south, suggesting that these neutrinos aren't a major problem for this detector.

Previously, the authors' analysis only picked up very high-energy events; Ernie and Bert were about one Peta-electronVolt each (for comparison, the LHC's protons are at 4 Tera-eV). Now, they've extended the sensitivity down to as low as 30TeV, with 28 events spread throughout the range of energies between the two. Seven of these produced muons in the detectors, indicating that they were produced by the muon neutrino. The rest produced a shower of signals, suggesting that they originated from some other form of neutrino.

The energies and properties involved in these neutrinos indicate that they originated outside our Solar System. Just as cosmic rays can produce neutrinos when they slam into something nearby, energetic events can produce neutrinos that travel significant distances across the Universe. One example might be if the jets of particles from a black hole slammed into a gas cloud, producing unstable particles like pions that decay in ways that produce neutrinos. Since all that energy ends up in a particle that's only a billionth of the mass of a proton, the neutrinos end up effectively traveling at the speed of light. Meaning that if we can see something anywhere in the Universe, we can also detect any neutrinos it produces.

The downside is that we don't yet have the ability to work backward to figure out the direction that the neutrinos originated from. We can give a rough area of the sky, but it's not good enough to direct observatories to image the source. At least within the IceCube detector, there was also no apparent pattern in time, indicating that it wasn't able to pick up any burst events. Although we're pretty sure these came from outside our Solar System, we can't currently say much about what produced them.

Science, 2013. DOI: 10.1126/science.1242856 (About DOIs).