On the Redshift of the Very High Energy Blazar 3C 66A

TL;DR

This paper uses ultraviolet spectra from HST to constrain the redshift of blazar 3C 66A, enabling better understanding of its gamma-ray emission and the extragalactic background light.

Contribution

It provides the first firm redshift lower and upper limits for 3C 66A based on UV absorption features, refining previous estimates.

Findings

Redshift lower limit: z > 0.3347

Redshift upper limit: z < 0.41 at 99% confidence

Rules out z > 0.444 at 99.9% confidence

Abstract

As a bright gamma-ray source, 3C 66A is of great interest to the high-energy astrophysics community, having a potential for placing cosmological constraints on models for the extragalactic background light (EBL) and the processes which contribute to this photon field. No firm spectroscopic redshift measurement has been possible for this blazar due to a lack of intrinsic emission and absorption features in optical spectra. We present new far-ultraviolet spectra from the Hubble Space Telescope/Cosmic Origins Spectrograph (HST/COS) of the BL Lac object 3C 66A covering the wavelength range 1132 to 1800 Angstroms . The data show a smooth continuum with intergalactic medium absorption features which can be used to place a firm lower limit on the blazar redshift of z>0.3347. An upper limit is set by statistically treating the non-detection of additional absorbers beyond z=0.3347, indicating a redshift of less than 0.41 at 99% confidence and ruling out z>0.444 at 99.9% confidence. We conclude by showing how the redshift limits derived from the COS spectra remove the potential for this gamma-ray emitting blazar to place an upper limit on the flux of the EBL using high energy data from a flare in October of 2009.