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

## TL;DR

This paper reclassifies TXS 0506+056 from a BL Lac to a masquerading BL Lac, a flat-spectrum radio quasar with hidden broad lines, impacting models of neutrino emission and blazar physics.

## Contribution

It provides evidence that TXS 0506+056 is a masquerading BL Lac, challenging previous classifications and assumptions about its emission properties and neutrino production.

## Key findings

- TXS 0506+056 is a masquerading BL Lac, not a true BL Lac.
- Its synchrotron peak frequency exceeds blazar sequence predictions.
- Reclassification affects models of neutrino emission and gamma-ray region location.

## Abstract

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.