In 2017, the Antarctic neutrino telescope IceCube detected a particle with an energy of about 290 TeV arriving from deep space. The source was identified as blazar TXS 0506+056—an active galactic nucleus at a redshift of z=0.3365, where a supermassive black hole sucks in matter from an accretion disk and ejects a narrow jet of plasma. This object became the first reliable association of a high-energy neutrino with an extragalactic source in the history of astrophysics. However, subsequent radio interferometric observations using VLBI techniques puzzled scientists: visible blobs of matter in the jet crawled at an apparent speed of only 1–2c — too slow to accelerate particles to such monstrous energy. A contradiction arose, dubbed the 'Doppler factor crisis.'

A fresh look at the 15-year observation archive by a team led by Yuri Kovalev revealed an unexpected picture. Abandoning the traditional method of identifying individual 'blobs,' the scientists analyzed the integrated brightness of the jet along its entire length, and discovered a coherent wave racing from the core at an apparent speed of (21 ± 1)c. This resembles the birth of a lightning bolt: an invisible stepped leader swiftly carves a path from cloud to ground, and only then does a blinding return stroke shoot upward. Similarly, in the jet: a hidden ultrarelativistic 'spine'—a stream of plasma with a Lorentz factor Γ>20—hardly emits in the radio band but carries a disturbance within it. The slow and luminous 'sheath' responds with a delay, producing the observed radio flare. It is precisely this motion of the spine that is registered as a wave traveling 21 times faster than light in a vacuum.

Superluminal motion is not a mirage but a geometric effect arising from Einstein's special theory of relativity and Lorentz's transformations: when plasma speeds almost along the line of sight, its apparent velocity can exceed c by many times. The formula for apparent speed takes into account both the true speed and the angle, while the Lorentz factor sets the scale of relativistic time dilation — a key ingredient explaining the nearly constant duration of the wave (~2 years) over several parsecs. In the spine's own reference frame, those two years compress to just a month and a half — hence the wave doesn't have time to dissipate, maintaining remarkable coherence.

The stratified model also naturally explains the three-year delay of the radio flux relative to the neutrino flare: neutrinos are born in the spine almost instantly, while the radio emission lights up later, when the transverse disturbance reaches the sheath. Plasma compression and likely magnetic reconnection in this transition zone create a bright cone visible to radio telescopes. The discovery resolves the long-standing crisis: standard kinematic analysis of VLBI jets based on individual components systematically underestimated the true flow speeds. Many active galactic nuclei may turn out to be far more powerful particle accelerators than previously thought, which is critical for interpreting data from neutrino observatories. Already it is clear that the spine-sheath scenario is universal: a second such wave, linked to a Baikal-GVD event, confirmed the reproducibility of the mechanism. Future studies with next-generation VLBI arrays (ngVLA, SKA) and millimeter telescopes (ALMA, Event Horizon Telescope) will allow direct resolution of the structure of these cosmic jets and may forever change our understanding of how nature builds its most extreme accelerators.