Neutrinos from outer space open new eye in the sky

Fancy seeing the sky in neutrino? Supermassive black holes and enormous stellar explosions may give up their secrets now that neutrinos from space can be detected.

The South Pole IceCube neutrino observatory has seen a handful of ghostly high-energy neutrinos that almost certainly came from outer space, opening up the skies for neutrino astronomy.

"We are witnessing the birth of this field," says Dan Hooper, a theoretical astrophysicist at Fermilab in Batavia, Illinois, who is not a member of IceCube.

Until now, the only space neutrinos definitively detected came from the sun and a 1987 supernova explosion in the Large Magellanic Cloud.

Last month, the IceCube collaboration published news of the detection of two high-energy neutrinos, each with an energy of about one petaelectronvolt. These neutrinos, discovered by accident a year ago and nicknamed Bert and Ernie, prompted the collaboration to go back and look at their data in more detail.

Flavour shift

The new analysis, reported today at the IceCube Particle Astrophysics symposium at the University of Wisconsin-Madison, has raised the stakes.

IceCube, which monitors a cubic kilometre of ice at the South Pole, saw 26 more neutrinos of about 50 teraelectronvolts between May 2010 and May 2012. There is only a 0.004 per cent chance that these 28 detections are due to chance. "This is not a statistical fluctuation," says Francis Halzen the leader of the IceCube collaboration at the University of Wisconsin-Madison.

Up to half of the observed events could be so-called atmospheric neutrinos, produced when cosmic rays smash into the upper atmosphere, but the rest must be coming from outside our solar system, according to IceCube team member Thomas Gaisser of the University of Delaware in Newark.

A key indication of this is the distribution of the different types of neutrinos. Neutrinos come in three flavours: muon, tau and electron. As they travel through space, they can change, or oscillate, from one type to another. Atmospheric processes produce twice as many muon neutrinos as electron neutrinos. However, the events detected by IceCube suggest that the different types are coming in equal numbers. "This is typical of a neutrino beam that has oscillated over a very long distance," says Halzen.

Microquasar probe

Being chargeless, neutrinos zip from a source direct to Earth without being waylaid. This makes them useful for investigating supernovae, mysterious objects called microquasars and active galactic nuclei – galaxies powered by supermassive black holes, all of which are thought to produce neutrinos.

"If you have something that looks into the hearts of these beasts, maybe it'll help untangle what's going on," says John Learned at the University of Hawaii, who is not part of IceCube.

Neutrino astronomy could also pinpoint the sources of cosmic rays, which are thought to be produced by the same processes as neutrinos. Because they are charged, cosmic rays get bent by intervening magnetic fields. This has made it impossible to work out their source by the time they reach Earth.

"IceCube being able to see the sky in neutrinos for the first time is absolutely going to transform how we view cosmic ray physics," says Hooper.

Dark direction

Neutrinos are also expected to be produced in regions such as the centre of the Milky Way, where dark matter particles are thought to accumulate in large numbers and smash into each other. Now that IceCube has shown it can detect high-energy astrophysical neutrinos, the next step is to work out their direction. Neutrinos with a specific energy coming from the galactic centre would be an indirect detection of dark matter.

While the latest IceCube neutrino detections are too few to provide information about their direction, Halzen says that his team is sitting on a goldmine of unanalysed data collected after May 2012. "We will have lots of events to work with, and we will figure this out, if not this year then next year," he says. "You cannot imagine the excitement in the collaboration."

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