Can an Optically Thick Circumstellar Shell Hide the Source of the Ultra-High-Energy Neutrino KM3-230213A? A Re-examination with Order-of-Magnitude Constraints

Revised version (October 2026). The previous version proposed that the KM3NeT event KM3-230213A came from a supernova at about 1 Gpc hidden behind a circumstellar shell with Thomson optical depth 10^3–10^5, and predicted a delayed infrared afterglow for JWST. This revision tests that hypothesis and finds it not viable for an isotropic source: (i) energetics — a source at 1 Gpc would need ≳2×10^54 erg in neutrinos near 220 PeV for a 10% chance of producing the event; with 10^51 erg it must lie within roughly 3–30 Mpc; (ii) acceleration versus trapping — shocks are radiation-mediated while the optical depth exceeds c/v ≈ 30, so particles can only be accelerated where photons escape on the dynamical time, not over years, and reaching the ~4 EeV proton energy required takes shock speeds of 2–3×10^4 km/s and a strong magnetic field; (iii) mass — the shells that would trap photons for years contain 40–400 solar masses. No LIGO–Virgo–KAGRA observing run was in progress on the date of the event, so the absence of a GW signal is uninformative. The JWST prediction of the previous version is withdrawn. The broader class of hidden neutrino sources (e.g. choked jets) remains viable but implies a nearby or strongly beamed source. All numbers are reproduced by the attached verify.py. References were checked; unverifiable references of the previous version were removed. AI assistance (Anthropic's Claude) was used and is disclosed in the manuscript.