Title: Looking into the Jet Cone of the Neutrino-Associated Very High Energy Blazar PKS 1424+240 Show Chinese title

Title: 查看与中微子相关的极高能耀变体PKS 1424+240的喷流锥

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 jet structure and magnetic field of the blazar PKS 1424+240, that has been selected on the basis of strong VHE gamma-ray emission and identified with one of the highest peaks in the IceCube 9-year neutrino sky. We analyze 15 GHz 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 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 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, indicating a current carrying jet flowing almost directly towards our line of sight. Blazars with very small jet viewing angles offer a solution to the longstanding mismatch between Doppler factors inferred from low VLBI apparent jet speed 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. Show Chinese abstract

Abstract: 大质量粒子的加速过程以及极高能（VHE）光子和中微子的产生仍然是天体物理学中的一个基本挑战。 我们研究了耀变体PKS 1424+240的角秒尺度喷流结构和磁场，该天体基于强烈的VHE伽马射线辐射被选中，并与IceCube 9年中中微子天空中的一个最高峰值相匹配。 我们分析了该BL Lac天体的15 GHz VLBA观测数据，将2009年至2025年间收集的42张偏振敏感图像叠加，以增强信号并揭示持续的角秒尺度结构。 我们的观测揭示了一个罕见的情景。 该天体位于喷流锥体内，非常接近其相对论性喷流的轴线，观测角度小于0.6度。 这有效地将多普勒增强提升到约30的值，并增强了观测者方向上的电磁辐射和中微子辐射。 根据偏振观测，我们明确检测到喷流磁场的净环向分量，表明一个携带电流的喷流几乎直接朝向我们的视线方向流动。 具有非常小喷流观测角度的耀变体为长期以来从低VLBI表观喷流速度推导出的多普勒因子与从VHE观测推导出的多普勒因子之间的不匹配问题提供了解决方案——即所谓的“多普勒因子危机”。 我们表明相对论束流在耀变体的伽马射线和中微子辐射中起着关键作用，这对它们的多信使辐射模型有直接意义。