Apparent superluminal core expansion and limb brightening in the
  candidate neutrino blazar TXS 0506+056
  IceCube has reported a very-high-energy neutrino (IceCube-170922A) in a
region containing the blazar TXS 0506+056. Correlated {\gamma}-ray activity has
led to the first high-probability association of a high-energy neutrino with an
extragalactic source. This blazar has been found to be in a radio outburst
during the neutrino event. We have performed target-of-opportunity VLBI imaging
observations at 43 GHz frequency with the VLBA two and eight months,
respectively, after the neutrino event. We produced two images of TXS 0506+056
with angular resolutions of (0.2x1.1) mas and (0.2x0.5) mas, respectively. The
source shows a compact, high brightness temperature core (albeit not
approaching the equipartition limit) and a bright and originally very
collimated inner jet. Beyond about 0.5 mas from the mm-VLBI core, the jet loses
this tight collimation and expands rapidly. During the months after the
neutrino event associated with this source, the overall flux density is rising.
This flux density increase happens solely within the core. The core expands in
size with apparent superluminal velocity during these six months so that the
brightness temperature drops by a factor of three in spite of the strong flux
density increase. The radio jet of TXS 0506+056 shows strong signs of
deceleration and/or a spine-sheath structure within the inner 1 mas
(corresponding to about 70 pc to 140 pc in deprojected distance) from the
mm-VLBI core. This structure is consistent with theoretical models that
attribute the neutrino and {\gamma}-ray production to interactions of electrons
and protons in the highly-relativistic jet spine with external photons
originating from a slower-moving jet region. Proton loading due to jet-star
interactions in the inner host galaxy is suggested as the possible cause of
deceleration
