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Monthly Notices of the Royal Astronomical Society Letters

We present evidence that TXS 0506+056, the first plausible non-stellar neutrino source, despite appearances, is not a blazar of the BL Lac type but is instead a masquerading BL Lac, i.e., intrinsically a flat-spectrum radio quasar with hidden broad lines and a standard accretion disk. This re-classification is based on: (1) its radio and O II luminosities; (2) its emission line ratios; (3) its Eddington ratio. We also point out that the synchrotron peak frequency of TXS 0506+056 is more than two orders of magnitude larger than expected by the so-called "blazar sequence", a scenario which has been assumed by some theoretical models predicting neutrino (and cosmic-ray) emission from blazars. Finally, we comment on the theoretical implications this re-classification has on the location of the $\gamma$-ray emitting region and our understanding of neutrino emission in blazars.

TXS 0506+056, the first cosmic neutrino source, is not a BL Lac