Origin of chromatic features in multiple quasars - Variability, dust, or microlensing

Aims. In some of lensed quasars, color differences between multiple images are observed at optical/near-infrared wavelengths. There are three possible origins for the color differences: intrinsic variabilities of quasars, differential dust extinction, and quasar microlensing. We examine how these possible scenarios can reproduce the observed chromaticity. Methods. We evaluate how much the color difference between multiple images can be reproduced by the scenarios with realistic models; (i) an empirical relation for intrinsic variabilities of quasars, (ii) empirical relations for dust extinction and theoretically predicted inhomogeneity in galaxies, or (iii) a theoretical model for quasar accretion disks and magnification patterns in the vicinity of caustics. Results. We find that intrinsic variabilities of quasars cannot be a dominant source responsible for observed chromatic features in multiple quasars. In contrast, either dust extinction or quasar microlensing can reproduce the observed color differences between multiple images in most of the lensed quasars. Taking into account the time interval between observations in different wavebands in our estimations, quasar microlensing is a more realistic scenario to reproduce the observed color differences than dust extinction. All the observed color differences presented in this paper can be explained by a combination of these two effects, but monitoring observations at multiple wavebands are necessary to disentangle them.