Measuring time delays: II. Using observations of the unresolved flux and astrometry

Abstract

Lensed quasars and supernovae can be used to study galaxies' gravitational potential and measure cosmological parameters. The typical image separation of objects lensed by galaxies is of the order of 0.5 ′′ . Therefore, finding the ones with small separations, and measuring their time-delays using ground-based observations is challenging. We suggest a new method to identify lensed quasars and simultaneously measure their time-delays, using seeinglimited synoptic observations in which the lensed quasar images and the lensing galaxy are unresolved. We show that using the light curve of the combined flux, and the astrometric measurements of the center-of-light position of the lensed images, the lensed nature of a quasar can be identified, and its time-delay can be measured. We provide the analytic formalism to do so, taking into account the measurement errors and the fact that the power spectra of quasar light curves is red (i.e., the light curve is highly correlated). We demonstrate our method on simulated data, while its implementation to real data will be presented in future papers. Our simulations suggest that, under reasonable assumptions, the new method can detect unresolved lensed quasars and measure their time delays, even when the image separation is below 0.1 ′′ , or the flux ratio between the faintest and brightest images is as low as 0.03. Python and MATLAB implementations are provided. In a companion paper, we present a method for measuring the time delay using the combined flux observations. Although the flux-only method is less powerful, it may be useful in cases in which the astrometric information is not relevant (e.g., reverberation mapping).

Out of One, Many: Distinguishing Time Delays from Lensed Supernovae

“…A combination of these two approaches and crosschecks between their results offer the potential to deal with a broad range of transients. (While we focus on transients, combined lightcurves from quasars have been studied by, e.g., [23][24][25][26][27].) Future work will seek to improve further our approaches to classify various lensed transients and estimate their time delays.…”

Varstrometry for Off-nucleus and Dual sub-Kpc AGN (VODKA): Hubble Space Telescope Discovers Double Quasars

“…Direct imaging confirmation for most of these sub-pc binary candidates is not feasible even with very long baseline interferometry in radio due to strict angular resolution requirement. Shen et al (2019) and Hwang et al (2020) have proposed a new astrometric technique, varstrometry, with Gaia data (Gaia Collaboration et al 2018) to discover close (sub-arcsecond) dual/lensed quasars (also see Liu 2015;Springer & Ofek 2021). It takes advantage of the ubiquitous quasar variability, which in dual or lensed quasars results in an astrometric noise signature when the asynchronous variability from the two quasars causes centroid jitters in the unresolved system.…”