Physicists using the IceCube Neutrino Observatory at the South Pole have detected for the first time mysterious, high-energy neutrinos that originate outside of the Solar System.
“The IceCube Collaboration has announced the observation of 28 extremely high energy events that constitute the first solid evidence for astrophysical neutrinos from outside our Solar System,” said Dr Spencer Klein of Lawrence Berkeley National Laboratory, who is a member of the IceCube Collaboration and co-author of the paper reporting the discovery in the journal Science.
“These 28 events include two of the highest energy neutrinos ever reported, which have been named Bert and Ernie.”
Nearly massless subatomic particles called neutrinos can carry information about the workings of the highest-energy and most distant phenomena in the Universe. Billions of neutrinos pass through every square centimeter of the Earth every second, but the vast majority originate either in the Sun or in the Earth’s atmosphere. Far rarer are neutrinos from the outer reaches of our Galaxy or beyond, which have long been theorized to provide insights into the powerful cosmic objects where high-energy cosmic rays may originate: supernovas, black holes, pulsars, active galactic nuclei and other extreme extragalactic phenomena.
Neutrino researchers have been looking to the sky for decades to learn more about these elusive particles.
Until now, they have seen low-energy neutrinos that originate in Earth’s atmosphere, neutrinos from farther out within the Solar System, and neutrinos from one rare nearby supernova, known as 1987A.
The 28 neutrinos observed by IceCube – a particle detector made from one cubic kilometer of ice in Antarctica – are different. They are at a significantly higher energy level than those produced by the previously measured sources.They were found in data collected from May 2010 to May 2012.
In analyzing more recent data, the IceCube Collaboration discovered another event (dubbed Big Bird) that was almost double the energy of Bert and Ernie.
“Like most scientific discoveries, finding Big Bird was a combination of hard work and luck and it took place on the afternoon of my last day of work on IceCube,” said co-author Dr Lisa Gerhardt of the National Energy Research Scientific Computing Center.
“At first I was in disbelief, thinking there must be some other explanation for this enormous event. However, one-by-one alternate explanations were disproved until finally we knew that we had found the most energetic event in IceCube yet, most likely from an astrophysical neutrino. I was able to leave IceCube with a bang!”
As to the identity of the mysterious cosmic accelerators, the physicists said these early results from IceCube favor active galactic nuclei, the enormous particle jets ejected by a black hole after it swallows a star.
“The 28 events being reported are diffuse and do not point back to a source, but the big picture tends to suggest active galactic nuclei as the leading contender with the second leading contender being something we haven’t even thought of yet,” Dr Klein said.
Bibliographic information: IceCube Collaboration. 2013. Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector. Science, vol. 342, no. 6161; doi: 10.1126/science.1242856