2024MNRAS.535.2742B

The BL Lac object PKS 1717+177 has been identified as potential neutrino-emitting active galactic nuclei in the point source stacking analysis of ICECUBE data. We explore peculiarities in the morphology and kinematics of the jet and examine multiwavelength light curves for distinctive effects which might allow to pinpoint a likely neutrino generation mechanism. We re-modelled 34 high-resolution radio interferometric Very Long Baseline Array observations obtained at 15 GHz (between 1999 December 27 and 2023 May 3). A correlation and periodicity analysis of optical Katzman Automatic Imaging Telescope and Tuorla data, as well as for Fermi-LAT $\gamma$-ray data has been performed. The nuclear jet appears deflected and bent at about 0.5 mas distance from the radio core by an encounter with a dark, unseen object. The deviation of the jet evolves over 23.5 yr from a simple apparent bend into a significantly meandering structure with increasing amplitude: a zig-zag line. To our knowledge, this is the first time that the temporal evolution of a jet deviation can be traced. The turning point shifts with time and the jet seems to brighten up almost periodically at the point of deviation. The radio core as well as the jet contribute approximately equally to the total flux-density at 15 GHz. We discuss scenarios which could explain the complex jet bending and quasi-regular flaring. We propose that the jet could either be deflected by the magnetosphere of a second massive black hole, by the pressure gradient due to a circumnuclear dense cloud, or via gravitational lensing by an intervening black hole.