A growing astrophysical neutrino signal in IceCube now features a 2-PeV neutrino

Strong evidence for a very high energy neutrino flux of extraterrestrial origin was found in November 2013, and new data from IceCube now confirms the discovery. Once more, the Antarctic detector brings us still the highest energy neutrino ever observed. This 2-PeV neutrino event was detected by IceCube on Tuesday, December 4, 2012. It was dubbed “Big Bird.”

Nine more very high energy events in data from 2012-2013 are reported on in a new paper from the IceCube Collaboration submitted to Physical Review Letters today. When added to the previous 28 events found in the 2010-2012 period, they allow for rejecting a purely atmospheric origin hypothesis at the 5.7 sigma level. The observed flux is consistent with an isotropic and equal-flavor E–2 power-law spectrum, as expected for an astrophysical neutrino flux.

The search for neutrino sources, either as significant clusters of observed neutrinos or as neutrinos observed in correlation with known gamma-ray sources, yielded no significant evidence.

Big Bird is one of 37 very high energy events seen in IceCube to an expected background of about 8 cosmic ray muon events and 7 atmospheric neutrino events.

“The new results confirm what we have seen in the previous paper,” explains Claudio Kopper, a John Bahcall Fellow at the Wisconsin IceCube Particle Astrophysics Center (WIPAC). The shape of this neutrino flux, consistent with an E–2 power-law spectrum as predicted by cosmic-ray generation models, would also show some indication of a cutoff around 2 PeV. If this cutoff is found, it will provide additional hints for candidate cosmic-ray sources and astrophysical environments for neutrino production. However, an equivalent and somewhat unexpected explanation would be a softer spectrum, as predicted for the interactions of cosmic rays with gas around supernova remnants in our and other galaxies. Only additional statistics will allow us to distinguish between these two hypotheses.

Regarding the origin of this flux, the lack of correlation with the galactic plane and the high galactic latitudes of some events suggest an extragalactic component. The production of PeV neutrinos requires hadronic interactions of cosmic rays with energies of a few 10s of PeV, extending into the transition between galactic and extragalactic cosmic rays. Along similar lines, the best-fit flux level in the central energy range is similar to the Waxman-Bahcall bound for neutrino fluxes produced in all extragalactic cosmic ray accelerators if they are optically thin. Moreover, a local contribution from galactic accelerators would show up as arrival direction clustering towards galactic structures. Thus, “the data so far lends support to an interpretation of many individually dim sources, with an extragalactic component”, adds Jacob Feintzeig, a graduate student at UW-Madison, who is one of the corresponding authors of the paper. However, more data is needed to confirm (or reject) both the extragalactic and the cosmic-ray related origin.

“Although finding more PeV neutrinos with a new year of IceCube data was expected, finding another of these beautiful high-energy events is still exciting, especially since it happens to be the highest energy neutrino ever observed. This is why we named it Big Bird,” says Lisa Gerhardt, an IceCube researcher at Lawrence Berkeley National Laboratory. Lisa found this event when she was analyzing a small sample of the 2012-2013 IceCube data.

“Last year, we received the first high-energy neutrinos from the cosmos. We’re now sitting down to listen to what stories they have come here to tell,” says Nathan Whitehorn, an IceCube researcher and also a corresponding author of this paper.

IceCubers are anxiously awaiting results from other neutrino searches as well as additional data from coming years. The answers to the longstanding cosmic-ray conundrum are closer than ever.

Deposited energies of observed events with predictions. The hashed region shows uncertainties on the sum of all backgrounds. Muons (red) are computed from simulation to overcome statistical limitations in our background measurement and scaled to match the total measured background rate. Atmospheric neutrinos and uncertainties thereon are derived from previous measurements of both the π/K and prompt components of the atmospheric ν_μ spectrum. A gap larger than the one between 400 and 1000 TeV appears in 43% of realizations of the best-fit continuous spectrum.
Arrival directions of the events in galactic coordinates. Shower-like events are marked with + and those containing muon tracks with x. Event IDs match those in the catalog in the online supplement and are time ordered. The grey line denotes the equatorial plane. The color map shows the test statistic (TS) for the point source clustering test at each location. No significant clustering was observed.

+ Info “Observation of High-Energy Astrophysical Neutrinos in Three Years of IceCube Data,” IceCube Collaboration: M.G. Aartsen et al. Physical Review Letters 113 (2014) 101101, arXiv.org:1405.5303 journals.aps.org