Upstream neutrino production and delayed jet emission in the blazar GB6 J1542+6129

Journal article, 2026

Aims. We present a multimessenger case study of the blazar GB6 J1542+6129. We examined whether its multiwavelength behavior is consistent with neutrino production in a compact region near the central black hole or with the parsec-scale radio jet. Methods. We performed a multimessenger analysis combining ∼17 years of Fermi-LAT γ-ray data, including a 5% adaptively binned light curve and Bayesian block decomposition, with ∼14 years of VLBI/MOJAVE observations to derive the Doppler factor evolution of the radio core. These were compared to the temporal properties of a suspected IceCube neutrino flare with a duration of 147−25⁺¹¹⁰ days, enabling a direct test of spatial and causal connections between neutrino and electromagnetic emission regions. Results. The suspected neutrino flare appears to precede a γ-ray flare and a pronounced increase in the very long baseline (VLBI) core Doppler factor by up to ∼1 year. The duration of the post-flare γ-ray activity is comparable to that of the neutrino flare, which in our framework is consistent with both signatures originating from a single propagating disturbance whose temporal structure is preserved during the propagation, rather than being set by the light-crossing time of any single emission zone. The γ-ray spectral energy distribution remains consistent in shape in the full, flare, and post-flare intervals, indicating stable particle acceleration conditions. The temporal ordering, taken at face value, places the neutrino production site upstream of the VLBI core. Conclusions. The observations of GB6 J1542+6129 are consistent with a disturbance-driven multi-zone scenario in which neutrinos are produced in a compact photon-rich inner region upstream of the parsec-scale VLBI core, plausibly in the coronal region, while the same disturbance later enhances Doppler-boosted leptonic emission when it reaches the radio core. While the data alone do not strictly establish this scenario on a population level, they show how time-domain multi-messenger observations of a single active galactic nucleus can localize neutrino emission relative to the parsec-scale radio jet in a particular source.