Selected results from IceCube

Abstract

Neutrino astronomy saw its birth with the discovery by IceCube of a diffuse flux at energies above 60 TeV with intensity comparable to a predicted upper limit to the flux from extra-galactic sources of ultra-high energy cosmic rays (UHECRs). While such an upper limit corresponds to the case of calorimetric sources, in which cosmic rays lose all their energy into photo-pion production, the first statistically significant coincident observation between neutrinos and gamma rays was observed from a blazar of intriguing nature. A very-high-energy muon event, of most probable neutrino energy of 290 TeV for an $E^{-2.13}$ spectrum, alerted other observatories triggering a large number of investigations in many bands of the electromagnetic (EM) spectrum. A high gamma-ray state from the blazar was revealed soon after the event and in a follow-up to about 40 days. A posteriori observations also in the optical and radio bands indicated a rise of the flux from the TXS 0506+056 blazar. A previous excess of events of the duration of more than 100~d was observed by IceCube with higher significance than the alert itself. These observations triggered more complex modeling than simple one-zone proton synchrotron models for proton acceleration in jets of active galactic nuclei (AGNs) and more observations across the EM spectrum. A second piece of evidence was a steady excess of about 50 neutrino events with reconstructed soft spectrum in a sample of lower energy well-reconstructed muon events than the alert event. A hot spot was identified in a catalog of 110 gamma-ray intense emitters and starburst galaxies in a direction compatible with NGC 1068 with a significance of $2.9σ$. NGC 1068 hosts a mildly relativistic jet in a starburst galaxy, seen not from the jet direction but rather through the torus. This Seyfert II galaxy is at only 14.4~Mpc from the Earth. We discuss these observations.