[Paper Review] Origin of Chromatic Features in Multiple Quasars -Variability, Dust, or Microlensing -

Atsunori Yonehara, Hiroyuki Hirashita+1 more|ArXiv.org|Sep 11, 2007

Galaxies: Formation, Evolution, Phenomena70 references22 citations

TL;DR

This study investigates the origin of chromatic features—color differences between multiple images—in lensed quasars, evaluating three mechanisms: intrinsic quasar variability, dust extinction, and microlensing. Using empirical and theoretical models, it finds that intrinsic variability cannot explain the observed color differences, while both dust extinction and microlensing can, with microlensing being more plausible due to time-delay constraints, suggesting a combination of both effects is likely. - meta_description: Explores chromatic features in lensed quasars via variability, dust, or microlensing; microlensing is most plausible for observed color differences. - objective: - To determine the dominant physical origin of observed chromatic features in multiple-image quasars. - To assess whether intrinsic quasar variability, dust extinction, or microlensing can reproduce observed color differences. - To evaluate the plausibility of each mechanism using realistic astrophysical models. - To compare the time-delay constraints of observations with expected variability timescales. - method: - Empirically models intrinsic quasar color variability using observed luminosity-color relations. - Applies empirical dust extinction laws with inhomogeneous dust distributions in lens galaxies. - Uses theoretical models of quasar accretion disks and microlensing magnification patterns near caustics. - Performs 10⁵ Monte Carlo realizations of lensing configurations to derive statistical distributions of caustic scale lengths. - Computes cumulative probability distributions of caustic sizes (xₛ) as a function of convergence (κ), external shear (γ), and smooth matter fraction (κ_c/κ). - Compares predicted color differences from each mechanism with observed data, accounting for time delays between waveband observations. - research_questions: - Can intrinsic quasar variability alone explain observed chromatic features in multiple quasars? - To what extent can differential dust extinction reproduce the observed color differences between lensed quasar images? - Can quasar microlensing produce the observed chromaticity, and how does it compare to dust extinction in plausibility? - How do time delays between observations at different wavelengths affect the interpretation of chromatic features? - Can a combination of dust extinction and microlensing explain all observed color differences? - key_findings: - Intrinsic quasar variability cannot account for the observed chromatic features, as its timescales are too slow to match the observed color differences. - Dust extinction can reproduce observed color differences, but requires fine-tuned dust distributions and extinction laws. - Quasar microlensing is a more realistic explanation due to its intrinsic wavelength dependence and compatibility with time-delayed observations. - The median caustic scale length (xₛ) in microlensing models is typically between 0.1 and 1 in units of the Einstein radius, with a minimum of ~10⁻⁵ r_E for extreme configurations. - The distribution of xₛ is most sensitive to the fraction of smooth matter (κ_c/κ), with less dependence on κ and γ. - All observed color differences can be explained by a combination of microlensing and dust extinction, but multi-wavelength monitoring is required to disentangle the two.

ABSTRACT

Aims:In some of the lensed quasars, color differences between multiple images are observed at optical/near-infrared wavelengths. There are three possible origins of the color differences: intrinsic variabilities of quasars, differential dust extinction, and quasar microlensing. We examine how these three possible scenarios can reproduce the observed chromaticity. Methods:We evaluate how much color difference between multiple images can be reproduced by the above three possible 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 nicely reproduce the observed color differences between multiple images in most of the lensed quasars. Taking into account the time interval between observations at 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 these.

Motivation & Objective

• To determine the dominant physical origin of observed chromatic features in multiple-image quasars.
• To assess whether intrinsic quasar variability, dust extinction, or microlensing can reproduce observed color differences.
• To evaluate the plausibility of each mechanism using realistic astrophysical models.
• To compare the time-delay constraints of observations with expected variability timescales.

Proposed method

• Empirically models intrinsic quasar color variability using observed luminosity-color relations.
• Applies empirical dust extinction laws with inhomogeneous dust distributions in lens galaxies.
• Uses theoretical models of quasar accretion disks and microlensing magnification patterns near caustics.
• Performs 10⁵ Monte Carlo realizations of lensing configurations to derive statistical distributions of caustic scale lengths.
• Computes cumulative probability distributions of caustic sizes (xₛ) as a function of convergence (κ), external shear (γ), and smooth matter fraction (κ_c/κ).
• Compares predicted color differences from each mechanism with observed data, accounting for time delays between waveband observations.

Experimental results

Research questions

• RQ1 Can intrinsic quasar variability alone explain observed chromatic features in multiple quasars?
• RQ2 To what extent can differential dust extinction reproduce the observed color differences between lensed quasar images?
• RQ3 Can quasar microlensing produce the observed chromaticity, and how does it compare to dust extinction in plausibility?
• RQ4 How do time delays between observations at different wavelengths affect the interpretation of chromatic features?
• RQ5 Can a combination of dust extinction and microlensing explain all observed color differences?

Key findings

• Intrinsic quasar variability cannot account for the observed chromatic features, as its timescales are too slow to match the observed color differences.
• Dust extinction can reproduce observed color differences, but requires fine-tuned dust distributions and extinction laws.
• Quasar microlensing is a more realistic explanation due to its intrinsic wavelength dependence and compatibility with time-delayed observations.
• The median caustic scale length (xₛ) in microlensing models is typically between 0.1 and 1 in units of the Einstein radius, with a minimum of ~10⁻⁵ r_E for extreme configurations.
• The distribution of xₛ is most sensitive to the fraction of smooth matter (κ_c/κ), with less dependence on κ and γ.
• All observed color differences can be explained by a combination of microlensing and dust extinction, but multi-wavelength monitoring is required to disentangle the two.