Further limits on the GRB contribution to astrophysical neutrinos and ultra-high-energy cosmic rays

The search for neutrino emission from gamma-ray bursts (GRBs) with IceCube continues. Previous analyses using track-like light signals induced by the interaction of muon neutrinos from the Northern Hemisphere sky have constrained emission from GRBs to below 1% of the observed astrophysical neutrino flux in IceCube.

Today, the IceCube Collaboration announces a new search for neutrino emission from GRBs with a first-ever search that covers all flavors and the full sky. Researchers looked for shower-like light patterns in IceCube produced by the interaction of electron, muon, and tau neutrinos using three years of data (May 2010 – May 2013). Five events were found to have a low-significance correlation with five GRBs. Consequently, the analysis places tight constraints on current models of neutrino and ultra-high-energy cosmic ray (UHECR) production in GRBs. These results have just been submitted to The Astrophysical Journal.

GRB all flavor
Exclusion contours, calculated from the combined three-year all-sky electron, muon, and tau neutrino shower-like event search and four-year Northern Hemisphere muon neutrino track-like event search results, of a per-flavor double broken power law GRB neutrino flux (see normalization in the paper). The upper marker is the flux expected from cosmic ray escape via neutrons. The lower marker is the flux expected from cosmic ray escape via protons. Credit: IceCube Collaboration

GRBs are abundant transient sources. Every day a new GRB is observed by spacecraft detectors dedicated to witnessing the brightest electromagnetic explosions in the universe. GRBs have been found to be distributed isotropically in the sky and to display a variety of gamma-ray emissions lasting from milliseconds to hours. Due of their extreme conditions, astrophysicists have long considered them the most plausible sources of the UHECRs that bombard our planet. IceCube can test this hypothesis because wherever these cosmic charged particles are accelerated, very high energy neutrinos inevitably are generated as well, and they must point back to their sources.

Researchers looked for emission from 807 GRBs reported during the data-taking period. They looked for all-flavor neutrino-induced showers in IceCube correlated with any of these GRB sources. Neutrinos change identities as they travel, as was confirmed by last year’s recipients of the Nobel Prize in Physics, and we expect all three neutrino types to show up in an astrophysical flux. Although five cascade neutrino events showed correlations with five different GRBs, their significance was low. Using these results along with those of previous searches using muon-neutrino induced tracks in IceCube, the study places the strongest limits yet on models of neutrino emission in GRBs.

Two classes of GRB neutrino emission models are considered: those normalized to the observed UHECR flux, which assume that GRB-accelerated protons are the dominant sources of the highest energy cosmic rays, and those normalized to the observed gamma-ray flux, which allow limits to be placed on internal model parameters. As shown in the image above, this analysis rules out the scenario of cosmic ray protons escaping the GRB as neutrons and then decaying back to protons outside of the magnetically confining fireball. Some recent models of high-energy protons directly escaping the fireball are still allowed, and ongoing analyses in combination with this study will probe their parameter spaces further.

The absence of high-energy neutrino emission from GRBs over multiple years of searches with IceCube has reshaped our understanding of these cataclysmic explosions and proven false the hypothesis that they are the dominant sources of UHECRs. “This study represents the widest net yet cast in the search for high-energy neutrinos from GRBs and has strengthened the surprising conclusion that these phenomena are not the cosmic ray generators the community long thought they were,” notes corresponding author Robert Hellauer, a recent PhD graduate at the University of Maryland. Continued analysis of astrophysical neutrino signals in IceCube will lead us to the origins of these still mysterious UHECR signals.

+ info “An All-Sky Search for Three Flavors of Neutrinos from Gamma-Ray Bursts with the IceCube Neutrino Observatory,” IceCube Collaboration: M.G.Aartsen et al, The Astrophysical Journal 824 (2016) 2, iopscience.iop.org arxiv.org/abs/1601.06484