The Redshift of the BL Lac Object TXS 0506+056

The bright BL Lac object TXS 0506+056 is the most likely counterpart of the IceCube neutrino event EHE 170922A. The lack of this redshift prevents a comprehensive understanding of the modeling of the source. We present high signal-to-noise optical spectroscopy, in the range 4100–9000 A, obtained at the 10.4 m Gran Telescopio Canarias. The spectrum is characterized by a power-law continuum and is marked by faint interstellar features. In the regions unaffected by these features, we found three very weak (EW ~ 0.1 A) emission lines that we identify with [O ii] 3727 A, [O iii] 5007 A, and [N ii] 6583 A, yielding the redshift z = 0.3365 ± 0.0010.