Letter to the Editor

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

¹ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, D–53121 Bonn, Germany
² Crimean Astrophysical Observatory, 298409 Nauchny, Crimea
³ Lebedev Physical Institute of the Russian Academy of Sciences, Leninsky Prospekt 53, 119991 Moscow, Russia
⁴ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁵ Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
⁶ Department of Physics and Astronomy, Purdue University, 525 Northwestern Avenue, West Lafayette IN 47907, USA
⁷ Black Hole Initiative, Harvard University, 20 Garden St, Cambridge, MA 02138, USA
⁸ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, FI-02540 Kylmälä, Finland
⁹ Aalto University Department of Electronics and Nanoengineering, PL 15500, FI-00076 Aalto, Finland
¹⁰ Institute for Nuclear Research, 60th October Anniversary Prospect 7a, Moscow 117312, Russia
¹¹ Physics Department, Lomonosov Moscow State University, 1-2 Leninskie Gory, Moscow 119991, Russia

^⋆ Corresponding author: yykovalev@gmail.com

Received: 6 May 2025
Accepted: 7 July 2025

Abstract

Context. 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.

Aims. We investigate the parsec-scale jet structure and magnetic field of the blazar PKS 1424+240, 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.

Methods. 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.

Results. 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°. 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.

Conclusions. 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.