Looking into the Jet Cone of the Neutrino-Associated Very High Energy Blazar PKS 1424+240

Abstract: 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 properties of the blazar PKS 1424+240, that has been selected on the basis of both gamma-ray and neutrino VHE radiation. We analyze VLBA observations of this BL Lac object, stacking 42 polarization-sensitive images collected in 2009-2025 to enhance the signal and reveal persistent parsec-scale structure. Our observations indicate that this object is viewed inside the jet cone, very close to the axis of its relativistic jet, with a viewing angle of <0.6 deg. This effectively maximizes Doppler boosting to values ~30 and enhances both electromagnetic and neutrino emission in the observer's direction. Based on polarimetric observations, we unambiguously detect a net toroidal component of the jet's magnetic field. Blazars with very small jet viewing angles offer a solution to the longstanding mismatch between Doppler factors inferred from VLBI and those derived from VHE observations -- the so-called `Doppler factor crisis'. We show that relativistic beaming plays the critical role in the gamma-ray and neutrino emission of blazars, with direct implications for models of their multi-messenger emission.