A Targeted Search for Variable Gravitationally Lensed Quasars

TL;DR

This paper introduces a novel pipeline that combines photometric variability analysis with traditional methods to identify gravitationally lensed quasars, successfully discovering new candidates in the DESI Legacy Imaging Surveys.

Contribution

The study presents a modified pipeline that incorporates variability detection via difference imaging, enhancing the identification of lensed quasars beyond morphology and color-based methods.

Findings

Identified 13 new gravitationally lensed quasar candidates from 5807 strong lens systems.

Detected significant variability in 13 known lensed quasars, including one quadruply-lensed system.

Demonstrated the effectiveness of variability-based search strategies in targeted surveys.

Abstract

We present a pipeline to identify photometric variability within strong gravitationally lensing candidates, in the DESI Legacy Imaging Surveys. In our first paper (Sheu et al. 2023), we laid out our pipeline and presented seven new gravitationally lensed supernovae candidates in a retrospective search. In this companion paper, we apply a modified version of that pipeline to search for gravitationally lensed quasars. From a sample of 5807 strong lenses, we have identified 13 new gravitationally lensed quasar candidates (three of them quadruply-lensed). We note that our methodology differs from most lensed quasar search algorithms that solely rely on the morphology, location, and color of the candidate systems. By also taking into account the temporal photometric variability of the posited lensed images in our search via difference imaging, we have discovered new lensed quasar candidates. While variability searches using difference imaging algorithms have been done in the past, they are typically preformed over vast swathes of sky, whereas we specifically target strong gravitationally lensed candidates. We also have applied our pipeline to 655 known gravitationally lensed quasar candidates from past lensed quasar searches, of which we identify 13 that display significant variability (one of them quadruply-lensed). This pipeline demonstrates a promising search strategy to discover gravitationally lensed quasars in other existing and upcoming surveys.