Looking into the jet cone of the neutrino-associated very high-energy blazar PKS 1424+240

The acceleration process of massive particles as well as the production of very high-energy (VHE) photons and neutrinos remains a fundamental challenge in astrophysics. We investigate the parsec-scale jet structure and magnetic field of the blazar which was selected on the basis of strong VHE gamma-ray emission and is identified with one of the highest peaks in the IceCube 9-year neutrino sky. We analyzed 15 GHz VLBA observations of this BL Lac object by stacking 42 polarization-sensitive images collected in 2009--2025 to enhance the signal and reveal the persistent parsec-scale structure. Our observations uncover a rare scenario. The object is viewed inside the jet cone, very close to the axis of its relativistic jet, with a viewing angle of <0.6^̧irc. This effectively maximizes Doppler boosting to values ∼30 and enhances the electromagnetic and neutrino emission in the direction of the observer. Based on polarimetric observations, we unambiguously detect a net toroidal component in the magnetic field of the jet. This indicates a current-carrying jet that flows almost directly toward our line of sight. Blazars with very small jet viewing angles offer a solution to the Doppler factor crisis, i.e., to the longstanding mismatch between Doppler factors inferred from the low apparent jet speed in very long-baseline interferometry and those derived from VHE observations. We show that relativistic beaming plays the critical role in the gamma-ray and neutrino emission of blazars. This has direct implications for models of their multimessenger emission.