The redshift of the BL Lac object TXS 0506+056

The bright BL Lac object TXS 0506+056 is a 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 $\mathring{\text{A}}$, obtained at the 10.4m 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 $\sim$ 0.1 $\mathring{\text{A}}$) emission lines that we identify with [O II] 3727 $\mathring{\text{A}}$, [O III] 5007 $\mathring{\text{A}}$, and [N II] 6583 $\mathring{\text{A}}$, yielding the redshift z = 0.3365$\pm$0.0010.