Origin of chromatic features in multiple quasars^*

Variability, dust, or microlensing

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstraße 12-14, Heidelberg 69120, Germany
² Department of Physics, Faculty of Science, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan e-mail: yonehara@cc.kyoto-su.ac.jp
³ Center for Computational Science, University of Tsukuba, Tennodai 1-1-1, Tsukuba, Ibaraki 305-8577, Japan e-mail: hirasita@ccs.tsukuba.ac.jp
⁴ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
⁵ Institut für Physik, Universität Potsdam, Am Neuen Palais 10, 14469 Potsdam, Germany e-mail: prichter@astro.physik.uni-potsdam.de

Received: 22 December 2006
Accepted: 5 September 2007

Abstract

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.