The blazar PKS 0605-085 as the origin of the KM3-230213A neutrino event

The origin of the remarkable ultra high energy neutrino event KM3-230213A observed with the ARCA detector is still unclear. In particular, most galactic scenarios are excluded, and at least some cosmogenic neutrino models are disfavoured. We show that the neutrino causing the KM3-230213A event could have been produced in the blazar PKS 0605-085 (redshift z = 0.87). This flat-spectrum radio quasar (FSRQ) is located at 2.4$^{\circ}$ from the reconstructed direction of the KM3-230213A event, allowing for an angular association between the blazar and the event in view of a significant direction measurement systematic uncertainty of 1.5$^{\circ}$ reported by the KM3Net Collaboration. FSRQs are believed to contain several distinct regions filled with photon fields external to the main relativistically moving particle acceleration zone (the “blob”), greatly boosting the neutrino production efficiency w.r.t. the synchrotron self-Compton (SSC) scenario. In particular, we consider a scenario where the external photon field is provided by the outer layer (the “sheath”) of the “spine-sheath” jet structure, and particle acceleration occurs inside the same blobs that are responsible for the 2021-2023 gamma-ray flare of the blazar PKS 0605-085. Furthermore, we note that the X-ray and gamma-ray constraints on the produced neutrino intensity could be significantly relaxed in the framework of the external photon field scenario. Finally, we make a verifiable prediction that after the planned re-calibration of the ARCA detector the new, refined direction of the KM3-230213A event will point to the blazar PKS 0605-085.