Observing the inner parsec-scale region of candidate neutrino-emitting
  blazars
  Context. Many questions on the nature of astrophysical counterparts of
high-energy neutrinos remain unanswered. There is increasing evidence of a
connection between blazar jets and neutrino events, with the flare of the
gamma-ray blazar TXS0506+056 in spatial and temporal proximity of IC170922A
representing one of the most outstanding associations of high-energy neutrinos
with astrophysical sources reported so far. Aims. With the purpose of
characterising potential blazar counterparts to high-energy neutrinos, we
analysed the parsec-scale regions of gamma-ray blazars in spatial coincidence
with neutrinos detected by IceCube. Specifically, we intended to investigate
peculiar radio properties of the candidate counterparts related to neutrino
production, as radio flares coincident to the neutrino detection or features in
jets morphology (limb brightening, transverse structures). Methods. We
collected multi-frequency VLBI follow-up observations of candidate counterparts
of four high-energy neutrino events detected by IceCube between January 2019
and November 2020. We analysed their radio characteristics soon after the
neutrino arrival in comparison with archival VLBI observations and
low-frequency radio observations. We discussed our results with respect to
previous statistical works and studies on the case of TXS 0506+056. Results. We
identified and analysed in detail five potential neutrino emitting blazars. Our
results suggest an enhanced state of radio activity for one source,
PKS1725+123. However, the lack of adequate monitoring prior to the neutrino
events was a limitation in tracing radio activity and morphological changes in
all the sources. Conclusions. We suggest that PKS1725+123 is a promising
neutrino source candidate. For the other four sources, our results alone do not
allow us to reveal a strong connection between the radio activity state at
neutrino arrival.
