Reverberation mapping and lensed quasars time delay using

Reverberation mapping and lensed quasars time delay using ULTRASAT Eran Ofek & Ofer Springer, Weizmann Institute

Measuring time delays • Can we detected lensed quasars and measure their time delays using synoptic survey when the lensed images are not resolved? • Can we measure reverberation time delays of quasars using a single-band synoptic observations?

Reverberation mapping • Observables: • Combined flux of all images • Can we use it to tell if the combined flux is composed of a linear combination of a time shifted light curve with itself? • Answer: NO • But YES if we have a statistical model of the light curve + the light curve has power over broad-band of frequencies.

Reverberation mapping

Lensed Quasars • Observables: • Combined flux of all images • Center of mass astrometric position • Can be used these to recover the source light curve? • Complications: • Quasars light curves are generated from a process which have very steep (red) power-spectrum. • We can use the observables to identify lensed quasars and simultaneously measure their time delay.

Lensed Quasars One can separate the observables (total flux, center of mass) from the unknowns (flux ratio, positions, power-spectrum)

Lensed Quasars • However, in reality, there are some complications: • There are measurement errors • The light curves of quasars are highly-correlated (steep, red powerspectrum). • Ignoring this facts: (1) will bias the results; (2) underestimate the uncertainties • One can derive a full statistical model…

Lensed Quasars • Works very well on simulations:

Summary • A new method to identify unresolved lensed quasars and simultaneously measure their time delay. • Requires >100 photometric and astrometric observations • Required precision: 50 mmag precision and 30 mas • A new method to measure the reverberation time delay from single broad-band observations • Requires >500 observations • A 2/A 1>0. 05 • Variability: >20%, precision <2% • Datasets: TESS, ZTF, LAST, ULTRASAT • Two methodology papers are being written: Springer & Ofek

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