The redshift of the BL Lac object TXS 0506+056

TL;DR

This paper determines the redshift of the BL Lac object TXS 0506+056 at z=0.3365 using high-quality optical spectroscopy, enabling better modeling of its role as a neutrino source.

Contribution

First spectroscopic measurement of TXS 0506+056's redshift, providing crucial data for understanding its physical properties and neutrino association.

Findings

Redshift of 0.3365 ± 0.0010 determined

Detected faint emission lines of [O II], [O III], and [N II]

Spectroscopy confirms the source's distance and aids in modeling

Abstract

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 $\textrm{\AA}$, 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 $\textrm{\AA}$) emission lines that we identify with [O II] 3727 $\textrm{\AA}$, [O III] 5007 $\textrm{\AA}$, and [N II] 6583 $\textrm{\AA}$, yielding the redshift z = 0.3365$\pm$0.0010.