The dynamics of the parsec-scale jet in the neutrino blazar PKS 0735+178

¹ Department of Physics, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany

^⋆ Corresponding author: jaeyoungkim@unist.ac.kr

Received: 4 September 2024
Accepted: 20 May 2025

Abstract

Recent studies of individual track-like TeV-PeV IceCube neutrino events suggest that strongly jetted active galactic nuclei (AGNs), that is, blazars, can be plausible sources of extragalactic high-energy neutrinos. Although the broadband emission and neutrinos from these blazars can be modeled by hadronic jets with inverse Compton processes, the various models show degeneracies. One of the reasons is the lack of high-resolution observations that determine the location and physical conditions of neutrino-emitting plasma. We present a very long-baseline interferometry (VLBI) study of PKS 0735+178, which was recently associated with the high-energy neutrino event IceCube-211208A as well as with alerts from other neutrino observatories. We analyzed publicly available Very Long Baseline Array (VLBA) 15 and 43 GHz data of PKS 0735+178 during 2020-2024, resolved the milliarcsecond-scale jet, and traced its time evolution in flux and structure before and after the IceCube-211208A event. We find significant enhancements in the radio flux density (of about a few times compared to its last quiescent state), apparent brightness temperature (∼×10), and synchrotron opacity at 15-43 GHz of the VLBI nuclear region (spectral index α₁₅⁴³ ∼ −0.34 to ∼+0.07) after IceCube-211208A. This strengthens the temporal correlation between the blazar and the high-energy neutrino event. Furthermore, we find that the source ejected a new VLBI component, C2, from the VLBI core before the IceCube-211208A event. C2 traveled farther downstream at an apparent speed of ∼4.2c, which is close to the historical maximum speed for this object. C2 then passed a subluminally moving feature in the jet, C1, which was located at ∼0.13 mas (∼0.77 pc) downstream of the core at the time of IceCube-211208A. The time of this apparent passage is statistically coincident with the time of IceCube-211208A within an uncertainty of 1σ. This suggests that the location of this apparent passage might be the probable spatial origin of the IceCube-211208A event. Examination of the kinematic and emission properties of C1 and C2 reveals that it is difficult to find a single unique jet model that explains the VLBI and the broadband emission properties of PKS 0735+178 simultaneously. Furthermore, we find that the large distance to C1 (∼0.77 pc) at the time of IceCube-211208A makes it difficult to provide a sufficient background radiation field for the photo-pion process in a single-zone jet model. Models that are less sensitive to the external radiation field are therefore preferred.