The parsec-scale jet of the neutrino-emitting blazar TXS~0506+056
  Recently the IceCube collaboration detected very high energy (VHE) neutrinos
and associated them with the blazar \txs{}, raising a possible association of
VHE neutrinos with this and other individual blazars. Very Long Baseline
Interferometry (VLBI) is so far the only technique enabling the imaging of the
innermost jet at milli-arcsec resolution (parsec scale), where the high energy
emission possibly originates from. Here, we report on the radio properties of
the parsec scale jet in \txs{} derived from the analysis of multi-epoch
multi-frequency archive VLBI data. The half opening angle of the jet beam is
about 3.8\degr, and the jet inclination angle is about 20\degr. The overall jet
structure shows a helical trajectory with a precessing period of 5--6 years,
likely originating from instabilities operating at parsec scales. The
calculated beaming parameters (Doppler boosting factor, bulk Lorentz factor)
suggest a moderately relativistic jet. The pc-scale magnetic field strength is
estimated in the contexts of core-shift and variability, and is in general
agreement in the range of 0.2 - 0.7 G. And it is found to decrease from a
relatively larger value during the quiescent period before the ongoing flare.
This suggests a conversion of magnetic field energy density to particle energy
density that help accelerate injected particles at the jet base and result in
variable shocked emission. The neutrino event could be associated with the
onset of energetic particle injection into the jet. This scenario then supports
the lepto-hadronic origin of the VHE neutrinos and $\gamma$-ray emission owing
to a co-spatial origin.
