What is the redshift of the gamma- ray BL Lac source S4 0954+65?

TL;DR

This study presents new spectroscopic data on the BL Lac object S4 0954+65, revealing its redshift remains uncertain and establishing a lower limit of z ≥ 0.45, which impacts understanding of its gamma-ray emission and the extragalactic background light.

Contribution

The paper provides the first high-quality spectroscopic observations that challenge previous redshift estimates and set a new lower limit for S4 0954+65.

Findings

Optical spectrum is featureless at an equivalent width limit of ~0.1 Angstrom.

Redshift is still unknown despite previous claims of z=0.367.

A new lower limit to the redshift is established at z ≥ 0.45.

Abstract

High signal-to-noise ratio spectroscopic observations of the BL Lac object S4 0954+65 at the alleged redshift z = 0.367 are presented. This source was detected at gamma frequencies by MAGIC (TeV) and FERMI (GeV) telescopes during a remarkable outburst that occurred in February 2015, making the determination of its distance particularly relevant for our understanding of the properties of the Extragalactic Background Light. Contrary to previous reports on the redshift, we found that the optical spectrum is featureless at an equivalent width limit of \sim 0.1 Ang. A critical analysis of the existing observations indicates that the redshift is still unknown. Based on the new data we estimate a lower limit to the redshift at z \geq 0.45.