A ring accelerator? Unusual jet dynamics in the IceCube candidate PKS
  1502+106
  On 2019/07/30.86853 UT, IceCube detected a high-energy astrophysical neutrino
candidate. The Flat Spectrum Radio Quasar PKS 1502+106 is located within the 50
percent uncertainty region of the event. Our analysis of 15 GHz Very Long
Baseline Array (VLBA) and astrometric 8 GHz VLBA data, in a time span prior and
after the IceCube event, reveals evidence for a radio ring structure which
develops with time. Several arc-structures evolve perpendicular to the jet
ridge line. We find evidence for precession of a curved jet based on kinematic
modelling and a periodicity analysis. An outflowing broad line region (BLR)
based on the C IV line emission (Sloan Digital Sky Survey, SDSS) is found. We
attribute the atypical ring to an interaction of the precessing jet with the
outflowing material. We discuss our findings in the context of a spine-sheath
scenario where the ring reveals the sheath and its interaction with the
surroundings (narrow line region, NLR, clouds). We find that the radio emission
is correlated with the $\gamma$-ray emission, with radio lagging the
$\gamma$-rays. Based on the $\gamma$-ray variability timescale, we constrain
the $\gamma$-ray emission zone to the BLR (30-200 $r_{\rm g}$) and within the
jet launching region. We discuss that the outflowing BLR provides the external
radiation field for $\gamma$-ray production via external Compton scattering.
The neutrino is most likely produced by proton-proton interaction in the blazar
zone (beyond the BLR), enabled by episodic encounters of the jet with dense
clouds, i.e. some molecular cloud in the NLR.
