What IceCube data tell us about neutrino emission from star-forming galaxies (so far)

Abstract

Very recently, the IceCube Collaboration reported a flux of neutrinos in the energy range $50\text{ }\text{ }\mathrm{TeV}\ensuremath{\lesssim}{E}_{\ensuremath{\nu}}\ensuremath{\lesssim}2\text{ }\text{ }\mathrm{PeV}$, which departs from expectations from atmospheric background at the $5.7\ensuremath{\sigma}$ level. This flux is in remarkable agreement with the expected diffuse flux of neutrinos from starburst galaxies, and the three highest energy events have uncertainty contours encompassing some of such systems. These events, all of which have well-measured energies above 1 PeV, exhibit shower topologies, for which the angular resolution is about 15\ifmmode^\circ\else\textdegree\fi{}. Due to this angular uncertainty and the a posteriori nature of cuts used in our study, it is not possible to assign a robust statistical significance to this association. Using muon tracks, which have angular resolution $<1\ifmmode^\circ\else\textdegree\fi{}$, we compute the number of observations required to make a statistically significant statement and show that in a few years of operation the upgraded IceCube detector should be able to confirm or refute this hypothesis. We also note that double bang topology rates constitute a possible discriminator among various astrophysical sources.