Observational information on high-energy astrophysical neutrinos is being continuously collected by the IceCube observatory. However, the sources of the neutrinos are still unknown. In this study, we use radio very long baseline interferometry (VLBI) data for a complete VLBI flux density–limited sample of active galactic nuclei (AGNs). We address the problem of the origin of astrophysical neutrinos with energies above 200 TeV in a statistical manner. It is found that AGNs positionally associated with IceCube events have typically stronger parsec-scale cores than the rest of the sample. The posttrial probability of a chance coincidence is 0.2%. We select the four strongest AGNs as highly probable associations: 3C 279, NRAO 530, PKS 1741−038, and OR 103. Moreover, we find an increase of radio emission at frequencies above 10 GHz around neutrino arrival times for several other VLBI-selected AGNs on the basis of RATAN-600 monitoring. The most pronounced example of such behavior is PKS 1502+106. We conclude that AGNs with bright Doppler-boosted jets constitute an important population of neutrino sources. High-energy neutrinos are produced in their central parsec-scale regions, probably in proton–photon interactions at or around the accretion disk. Radio-bright AGNs that are likely associated with neutrinos have very diverse γ-ray properties, suggesting that γ-rays and neutrinos may be produced in different regions of AGNs and not directly related. A small viewing angle of the jet–disk axis is, however, required to detect either of them.

Recently we have shown that high-energy neutrinos above 200 TeV detected by IceCube are produced within several parsecs in the central regions of radio-bright blazars, that is active galactic nuclei with jets pointing toward us. To independently test this result and extend the analysis to a wider energy range, we use public data for all neutrino energies from seven years of IceCube observations. The IceCube point-source likelihood map is analyzed against the positions of blazars from a statistically complete sample selected according to their compact radio flux density. The latter analysis delivers a 3.0σ significance, with the combined post-trial significance of both studies being 4.1σ. The correlation is driven by a large number of blazars. Together with fainter but physically similar sources not included in the sample, they may explain the entire IceCube astrophysical neutrino flux as derived from muon-track analyses. The neutrinos can be produced in interactions of relativistic protons with X-ray self-Compton photons in parsec-scale blazar jets.

Observational studies of collimation in jets in active galactic nuclei (AGN) are a key to understanding their formation and acceleration processes. We have performed an automated search for jet shape transitions in a sample of 367 AGN using VLBA data at 15 and 1.4 GHz. This search has found 10 out of 29 nearby jets at redshifts z < 0.07 with a transition from a parabolic to conical shape, while the full analysed sample is dominated by distant AGN with a typical z ≈ 1. The ten AGN are UGC 00773, NGC 1052, 3C 111, 3C 120, TXS 0815−094, Mrk 180, PKS 1514+00, NGC 6251, 3C 371, and BL Lac. We conclude that the geometry transition may be a common effect in AGN jets. It can be observed only when sufficient linear resolution is obtained. Supplementing these results with previously reported shape breaks in the nearby AGN 1H 0323+342 and M87, we estimate that the break occurs at 105–106 gravitational radii from the nucleus. We suggest that the jet shape transition happens when the bulk plasma kinetic energy flux becomes equal to the Poynting energy flux, while the ambient medium pressure is assumed to be governed by Bondi accretion. In general, the break point may not coincide with the Bondi radius. The observational data support our model predictions on the jet acceleration and properties of the break point.