Evidence for gravitational lensing in IceCube-211208A counterpart PKS 0735+178: neutrino lensing proposed

Abstract

Four independent Neutrino Detectors (IceCube, Baikal, KM3NeT, Baksan) reported events from the direction of PKS 0735+178 in 2021 December. We perform a thorough investigation of the nuclear jet in this outstanding, neutrino emitting AGN. 40 epochs of 15 GHz VLBA data (1995.27–2025.02) and 116 epochs of 43 GHz VLBA data (2008.62–2025.22), covering also the time of the neutrino detections, have been analysed. Our findings are best explained by gravitational lensing by a known intervening absorption system (z = 0.424), to bend the jet into loops and rings. In addition, we suggest that jet precession aligns the more nuclear parts of the jet with the lens. Combined gravitational lensing and jet precession gradually impact different parts of the jet to be lensed and curved into rings of different sizes with time. A de-lensed source reconstruction with a point-mass lens model ((Formula presented) few (Formula presented) M(Formula presented)) is applied. The pronounced curved and loop-like morphology observed in the image-plane distribution appear as an almost straight jet in the reconstructed source plane. Radio and (Formula presented) -ray variability correlate with times of loops and are most likely enhanced by gravitational lensing as well. All neutrinos are detected within the rising wing of the largest (Formula presented) -ray flare ever observed in PKS 0735+178. Gravitational lensing explains why this blazar is detected at this time as a neutrino emitter: a magnification factor of about 10 can move an otherwise sub-threshold neutrino flux above the alert threshold.