Imprints of the quasar structure in time-delay light curves: Microlensing-aided reverberation mapping

Abstract

Owing to the advent of large area photometric surveys, the possibility to use broad band photometric data, instead of spectra, to measure the size of the broad line region of active galactic nuclei, has raised a large interest. We describe here a new method using time-delay lensed quasars where one or several images are affected by microlensing due to stars in the lensing galaxy. Because microlensing decreases (or increases) the flux of the continuum compared to the broad line region, it changes the contrast between these two emission components. We show that this effect can be used to effectively disentangle the intrinsic variability of those two regions, offering the opportunity to perform reverberation mapping based on single band photometric data. Based on simulated light curves generated using a damped random walk model of quasar variability, we show that measurement of the size of the broad line region can be achieved using this method, provided one spectrum has been obtained independently during the monitoring. This method is complementary to photometric reverberation mapping and could also be extended to multi-band data. Because the effect described above produces a variability pattern in difference light curves between pairs of lensed images which is correlated with the time-lagged continuum variability, it can potentially produce systematic errors in measurement of time delays between pairs of lensed images. Simple simulations indicate that time-delay measurement techniques which use a sufficiently flexible model for the extrinsic variability are not affected by this effect and produce accurate time delays.

H0LiCOW – I. H0 Lenses in COSMOGRAIL's Wellspring: program overview

“…Regular and frequent observations, at least once every few days, are necessary so that the variability pattern of the background source can be observed in each of the multiple images and be matched up to obtain the time delays. Monitoring in the optical requires a long baseline or high photometric precision to overcome systematic variations due to microlensing by stars in the lensing galaxy that could be mistaken as the background source intrinsic variability (e.g., Tewes et al 2013b;Sluse & Tewes 2014). Curve-shifting methods have been developed to measure the time delays from the light curves (e.g., Press et al 1992;Pelt et al 1996;Fassnacht et al 2002;Harva & Raychaudhury 2008;Hirv et al 2011;Morgan et al 2008;Tewes et al 2013a;Hojjati et al 2013).…”

Double dark matter vision: twice the number of compact-source lenses with narrow-line lensing and the WFC3 grism

“…Continuum and broad emission features are all time variable as well as small enough to be differentially magnified by stars in the plane of the lens galaxy. This differential magnification by stars can affect not only the overall amplitude, but also the shapes of the continuum and broad emission, as bluer continuum light and higher velocity broad-line emission are emitted from systematically smaller regions which are therefore are more susceptible to microlensing (Abajas et al 2002;Keeton et al 2006;Anguita et al 2008;Mosquera & Kochanek 2011;Blackburne et al 2011;Sluse et al 2007Sluse et al , 2011Sluse et al , 2012Sluse & Tewes 2014;Blackburne et al 2014;Jiménez-Vicente et al 2014;Fian et al 2018;Bate et al 2018).…”

Time Delay Analysis of the Lensed Quasar SDSS J1001+5027

“…Apart from discussed statistical issues, microlensing is another concern in time delay estimation which can cause distortion in the light curves and hence systematic errors in time delay estimation (e.g. Tewes et al (2013a); Sluse & Tewes (2014)). In our analysis the light curves are compared in segments of data, considering that microlensing effect can be assumed as the linear distortion in different segments.…”