Using gravitational lensed images to investigate the intrinsic AGN variability

TL;DR

This paper demonstrates how gravitational lensing flux ratios can be used to study intrinsic AGN variability with high precision, revealing weak variability episodes and jet structure details through multi-frequency monitoring.

Contribution

It introduces a method to analyze intrinsic AGN variability using flux-density ratios of lensed images, with a case study on B0218+357 showing chromatic variability and jet opacity effects.

Findings

Detection of weak variability episodes in AGN flux ratios

Chromatic variability observed before flux increase

Jet opacity model explains flux ratio differences

Abstract

We discuss about how the relative flux densities among the images of gravitationally-lensed active galactic nuclei, AGN, can be used to study the intrinsic AGN variability with high accuracy. Multi-frequency monitoring observations of resolved gravitational lenses can allow us to detect signals of very weak variability and also provide information about the jet opacity and structure. As an example, we investigate the variability of the flux-density ratio between the two lensed images of the blazar B0218+357, using dual-frequency cm-wave observations. Similar to our previously reported submm-wave observations of the lensed blazar PKS1830-211, we observe a clear chromatic variability, starting short before an increase in the flux-density of the blazar. The evolution of the flux-density ratios between the blazar images shows a more clear and rich structure than that of the mere lightcurves of each individual image. The accuracy in the ratio measurements is allowing us to see variability episodes in the blazar that are weaker than the natural scatter in the absolute flux-density measurements. A simple opacity model in the jet is used to consistently explain the difference between the flux-density-ratio evolution at the two frequencies.