Discovering strongly lensed QSOs from unresolved light curves

Yiping Shu,Vasily Belokurov,N Wyn Evans

doi:10.1093/mnras/stab241

Yiping Shu, Vasily Belokurov + Show 1 more

Open Access

https://doi.org/10.1093/mnras/stab241

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Abstract

ABSTRACT We present a new method of discovering galaxy-scale, strongly lensed QSO systems from unresolved light curves using the autocorrelation function. The method is tested on five rungs of simulated light curves from the Time Delay Challenge 1 that were designed to match the light-curve qualities from existing, ongoing, and forthcoming time-domain surveys such as the Medium Deep Survey of the Panoramic Survey Telescope And Rapid Response System 1, the Zwicky Transient Facility, and the Rubin Observatory Legacy Survey of Space and Time. Among simulated lens systems for which time delays can be successfully measured by current best algorithms, our method achieves an overall true-positive rate of 28–58 per cent for doubly imaged QSOs (doubles) and 36–60 per cent for quadruply imaged QSOs (quads) while maintains ≲10 per cent false-positive rates. We also apply the method to observed light curves of 22 known strongly lensed QSOs, and recover 20 per cent of doubles and 25 per cent of quads. The tests demonstrate the capability of our method for discovering strongly lensed QSOs from major time domain surveys. The performance of our method can be further improved by analysing multifilter light curves and supplementing with morphological, colour, and/or astrometric constraints. More importantly, our method is particularly useful for discovering small-separation strongly lensed QSOs, complementary to traditional imaging-based methods.

Highlights

Lensed QSO systems have been shown to be a powerful tool for a broad range of important topics in astronomy and astrophysics
We set the maximum number of hits allowed to be five and show true positive rate (TPR) and false positive rate (FPR) for Time Delay Challenge 1 (TDC1) light curves with reference configurations that achieve the highest TPRs at FPRs 10 per cent as an example
We present a new method of discovering strongly lensed QSO systems using their joint light curves

Summary

Introduction

Lensed QSO systems have been shown to be a powerful tool for a broad range of important topics in astronomy and astrophysics They can be used to measure the initial mass function (IMF) and dark-matter substructures in the lensing galaxies (e.g., Mao & Schneider 1998; Metcalf & Madau 2001; Dalal & Kochanek 2002; Pooley et al 2009; Nierenberg et al 2014; Oguri et al 2014; Schechter et al 2014; Jiménez-Vicente & Mediavilla 2019). Utilising time delays from strongly lensed QSOs, independent and precise measurements of the Hubble constant have been achieved (e.g., Suyu et al 2013, 2014; Bonvin et al 2017; Wong et al 2020), which are of particular importance given the growing tension in the Hubble constant measured by different approaches (e.g., Addison et al 2018; Planck Collaboration et al 2018; Riess et al 2019; Macaulay et al 2019) that could hint for. This is because binary QSOs can have very similar optical spectra due to the generic nature of QSO spectra, sometimes even more similar than doublyimaged QSOs (e.g., Peng et al 1999; Mortlock et al 1999)

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