# On the redshift of TeV BL Lac objects

## TL;DR

This study used high-quality optical spectroscopy to determine or constrain the redshifts of 22 TeV-detected BL Lac objects, crucial for understanding their emission, population, and photon interactions with the background light.

## Contribution

It provides new redshift measurements and limits for several TeV BL Lac objects, improving the accuracy of their distance estimates and aiding high-energy astrophysics research.

## Key findings

- New redshift for PKS 1424+240 (z=0.604)
- Tentative redshift for 1ES 0033+595 (z=0.467)
- Redshift lower limits for three sources based on absorption features

## Abstract

We report results of a spectroscopic campaign carried out at the 10 m Gran Telescopio Canarias for a sample of 22 BL Lac objects detected (or candidates) at TeV energies, aimed to determine or constrain their redshift. This is of fundamental importance for the interpretation of their emission models, for population studies and also mandatory to study the interaction of high energy photons with the extragalactic background light using TeV sources. High signal-to-noise optical spectra in the range 4250 - 10000 {\AA} were obtained to search for faint emission and/or absorption lines from both the host galaxy or the nucleus. We determine a new redshift for PKS 1424+240 (z = 0.604) and a tentative one for 1ES 0033+595 (z = 0.467). We are able to set new spectroscopic redshift lower limits for other three sources on the basis of Mg II and Ca II intervening absorption features: BZB J1243+3627 (z > 0.483), BZB J1540+8155 (z > 0.672), and BZB 0J2323+4210 (z > 0.267). We confirm previous redshift estimates for four blazars: S3 0218+357 (z = 0.944), 1ES 1215+303 (z = 0.129), W Comae (z = 0.102), and MS 1221.8+2452 (z = 0.218). For the remaining targets, in seven cases (S2 0109+22, 3C 66A, VER J0521+211, S4 0954+65, BZB J1120+4214, S3 1227+25, BZB J2323+4210), we do not validate the proposed redshift. Finally for all sources of still unknown redshift, we set a lower limit based on the minimum equivalent width of absorption features expected from the host galaxy.