Neutrino and Photon Arrival Times from TXS 0506+056: A Blazar Coincidence Converted to a Speed Bound

On 22 September 2017 the IceCube detector recorded a neutrino of about 290 TeV whose direction matched the blazar TXS 0506+056, at redshift 0.3365, during an ongoing gamma-ray flare. Converting the coincidence into a speed comparison: over a lookback time of 3.88 billion years, association within the flare's roughly 158-day enhanced state bounds the fractional neutrino-photon speed difference at 1.12e-10, while a one-day coincidence — sharper than this event can claim — would reach 7.06e-13. The energy of the neutrino makes the event valuable despite the loose timing: it extends multi-messenger speed comparison to hundreds of TeV, a regime no gamma-ray burst photon reaches. The framework examined here assigns the neutrino the same vacuum speed as light up to its negligible mass correction; the coincidence is consistent with that statement. Computed from published data; not a new measurement.