A hidden ultra-relativistic spine in the jet of a neutrino-associated blazar
Supermassive black holes launch powerful jets of plasma that can accelerate particles to extreme energies, but the physical conditions required to produce high-energy neutrinos remain unknown. In the blazar TXS 0506+056, the first source individually linked to a high-energy neutrino, radio images had seemed to reveal a jet too slow to sustain the extreme conditions required for neutrino production. Here we resolve this tension using long-term radio monitoring, particularly Very Long Baseline Interferometry (VLBI) imaging. We uncover a disturbance in emission propagating with an apparent speed of 21+-1 times the speed of light, that is masked by slower, radio-bright features that dominated earlier analyses. We interpret this as the signature of a stratified jet: an ultra-relativistic spine with Lorentz factor Gamma>20 embedded within a slower outer sheath. As the disturbance travels along the spine, it progressively illuminates the sheath, producing the delayed radio flare and naturally accounting for the years-long offset between neutrino and radio emission. The same pattern recurs in a second neutrino-associated event, pointing to a repeatable multi-messenger engine. These findings challenge the standard interpretation of VLBI jet speeds and establish a concrete, testable framework connecting structured jets to the sources of the Universe's highest-energy neutrinos.