On the redshift of the blazar PKS0447-439

TL;DR

This study refutes previous claims of a high redshift for the blazar PKS0447-439 by analyzing high-quality spectra, showing the earlier absorption line was due to telluric contamination and not an extragalactic feature.

Contribution

The paper provides a new high signal-to-noise spectrum that challenges prior redshift estimates, clarifying the blazar's distance measurement.

Findings

Confirmed the presence of an absorption line at 6280A

Identified the line as telluric, not extragalactic

Could not determine the redshift of PKS0447-439

Abstract

PKS0447-439 is one of the BL Lacertae objects that have been detected at very high energy. There has been a recent report of a lower limit of z>1.246 for the redshift of this blazar, challenging the current paradigm in which very high-energy gamma-rays cannot freely propagate in the z>1 universe. In this research note, we present a new MagE/Magellan spectrum of PKS0447-439 with exquisite signal-to-noise (S/N>150 at 6500A). Our analysis confirms the presence of the previously-reported absorption line at 6280A, which we identify, however, with a known telluric absorption, invalidating the claim that this blazar lies at z>1. Since no other extragalactic spectral features are detected, we cannot establish a redshift based on our spectrum.