The Science paper is precise: a muon track detected September 22, 2017, an alert distributed worldwide within one minute, Fermi's LAT reporting on September 28 that the direction matched a cataloged gamma-ray source undergoing a flare, with MAGIC then finding gamma rays up to 400 GeV. IceCube's own archive held an earlier neutrino flare from the same blazar in 2014-15. The correlation held at three standard deviations.
Cosmic rays were detected in 1912 and immediately posed the question they still mostly refuse to answer: what launches them. Being charged, they get bent by every magnetic field between there and here, so their directions carry no address. Neutrinos from the same engines fly straight. The blazar paper was less a discovery than an interception.
The entire event, telescopes across the spectrum and around the planet repointing on a schedule, began with one muon track in the ice. Never before had so much instrumentation been turned by so little matter. The alert system made the detector a dispatcher, not just a recorder.
Cosmic rays were found in 1912 and their origin stayed hidden for a century because magnetic fields scramble their paths; what else is visible only to particles that never swerve?