Cosmic Neutrinos and the Cosmic-Ray Accelerator TXS 0506+056

IceCube discovered a flux of cosmic neutrinos originating in extragalactic sources with an energy density close to that in gamma rays and cosmic rays. A multimessenger campaign triggered by the coincident observation of a gamma-ray flare and a 290-TeV IceCube neutrino pinpointed the cosmic-ray accelerator TXS 0506+056. Subsequently, the IceCube archival data revealed a 3-month burst of 13 cosmic neutrinos in 2014-15 that dominates the neutrino flux of the source over the 9.5 years of observations. The original identification of the source as a blazar was puzzling because it requires a major accretion event onto the rotating supermassive black hole to accommodate the neutrino burst. Subsequent high-resolution radio images of the source with the VLBA brought to light a merger of two galaxies, revealed by the interaction of two jets entangled in the source. Recently, the blazar PKS 1502+106 was found in the direction of a 300-TeV neutrino alert, IC-190730. OVRA radio observations at 15 GHz indicate that the neutrino also coincides with the highest flux density of a flare that started five years ago. This matches the similar long-term outburst seen from TXS 0506+056 and may indicate merger activity. Also, the dominant hotspot in the 10-year IceCube neutrino sky map, NGC 1068 (Messier 77), is a Seyfert galaxy undergoing a major accretion event onto the black hole. A few-percent fraction of such special sources, now labeled as gamma-ray blazars, is sufficient to accommodate the diffuse cosmic neutrino flux observed by IceCube. While rapid progress seems likely, the observations also convincingly make the case for the construction of more and larger neutrino telescopes with better angular resolution.