The Relativistic Jet Orientation and Host Galaxy of the Peculiar Blazar PKS 1413+135

TL;DR

This study clarifies the jet orientation and host galaxy of the peculiar blazar PKS 1413+135, revealing it is a background object with a jet aligned close to our line of sight, challenging previous classifications.

Contribution

It provides the first definitive determination of the jet axis orientation and the true background nature of PKS 1413+135, resolving longstanding uncertainties.

Findings

Jet axis is aligned close to the line of sight.

PKS 1413+135 is a background object, not in the apparent host galaxy.

Intervening galaxy at z=0.247 is the lens and host galaxy.

Abstract

PKS 1413+135 is one of the most peculiar blazars known. Its strange properties led to the hypothesis almost four decades ago that it is gravitationally lensed by a mass concentration associated with an intervening galaxy. It exhibits symmetric achromatic variability, a rare form of variability that has been attributed to gravitational milli-lensing. It has been classified as a BL Lac object, and is one of the rare objects in this class with a visible counterjet. BL Lac objects have jet axes aligned close to the line of sight. It has also been classified as a compact symmetric object, which have jet axes not aligned close to the line of sight. Intensive efforts to understand this blazar have hitherto failed to resolve even the questions of the orientation of the relativistic jet, and the host galaxy. Answering these two questions is important as they challenge our understanding of jets in active galactic nuclei and the classification schemes we use to describe them. We show that the jet axis is aligned close to the line of sight and PKS 1413+135 is almost certainly not located in the apparent host galaxy, but is a background object in the redshift range $0.247 < z < 0.5$. The intervening spiral galaxy at $z = 0.247$ provides a natural host for the putative lens responsible for symmetric achromatic variability and is shown to be a Seyfert 2 galaxy. We also show that, as for the radio emission, a "multizone" model is needed to account for the high-energy emission.