[Paper Review] Multi-wavelength study of the gravitational lens system RXS J113155.4-123155: I. Multi-epoch optical and near infrared imaging

Dominique Sluse, Jean-François Claeskens+7 more|ArXiv.org|Dec 19, 2005

Galaxies: Formation, Evolution, Phenomena53 references14 citations

TL;DR

This study presents multi-epoch optical and near-infrared imaging of the gravitational lens system RXS J1131-123155 using HST, ESO, and CFHT data, applying the MCS deconvolution method to separate quasar point-source flux from its host galaxy and accurately measure flux variations. The key finding is that microlensing and a complex lens potential—including an octupole term—jointly explain the anomalous flux ratios, challenging the need for massive substructures in the lensing galaxy.

ABSTRACT

(abridged) Aims: RXS J113155.4-123155 (z=0.66) is a quadruply imaged lensed quasar with a resolved Einstein Ring. The goal of this paper is to provide a full characterization of this system, and more particularly accurate astrometry and photometry. Method: Visible and near-infrared imaging observations of RXS J113155.4-123155 were carried out at various epochs using several ground based telescopes and the HST. The frames have been deconvolved using the MCS algorithm. A Singular Isothermal Ellipsoid (SIE) + external shear has been used to model the lensing galaxy potential. Results: MCS deconvolution enables us to separate the flux of the QSO (point-like images) from that of its host galaxy and to accurately track the flux variations of the point-like images in various filters. The deconvolved frames unveil several multiply imaged structures in the Einstein ring and an unidentified object in the vicinity of the lensing galaxy. We discuss the lightcurves and the chromatic flux ratio variations and deduce that both intrinsic variability and microlensing took place during a span longer than one year. We demonstrate that microlensing may easily account for the so called anomalous flux ratios presented in the discovery paper. However, the observed flux ratios are still poorly reproduced when modeling the lens potential with a SIE+shear. We argue that this disagreement can hardly be explained by milli-lensing caused by substructures in the lensing galaxy. A solution proposed in Paper II consists in a more complex lens model including an octupole term to the lens gravitational potential.

Motivation & Objective

• To provide accurate astrometry and photometry of the quadruply imaged quasar RXS J1131-123155 for precise lens modeling.
• To investigate the origin of anomalous flux ratios in the Einstein ring system, particularly the discrepancy between observed flux ratios and predictions from standard SIE+shear models.
• To assess the role of microlensing and intrinsic quasar variability in flux ratio variations over time and across wavelengths.
• To determine whether the observed flux anomalies are better explained by substructures in the lensing galaxy or by higher-order multipole terms in the lens potential.
• To establish photometric accuracy limits in the presence of a bright Einstein ring, which complicates flux measurements of the point-like images.

Proposed method

• Acquired multi-epoch optical and near-infrared imaging using HST, ESO VLT, and CFHT telescopes across multiple filters.
• Applied the MCS (Maximum Curvature deconvolution) algorithm to deconvolve images, preserving photometry and enabling separation of point-source and extended source fluxes.
• Modeled the lens potential using a Singular Isothermal Ellipsoid (SIE) plus external shear, later extended with an octupole term in Paper II.
• Tracked flux variations of the four quasar images across filters and over time (Nov 2002–Apr 2004) to identify intrinsic variability and microlensing effects.
• Used the $R_{\rm{cusp}}$ relation (Mao & Schneider, 1998) to constrain the presence of small-scale structures and assess their impact on flux ratios.
• Quantified systematic photometric errors due to the superimposed Einstein ring, especially in the R band, and evaluated photometric accuracy near the photon noise limit.

Experimental results

Research questions

• RQ1 What causes the anomalous flux ratios in the quadruply imaged quasar RXS J1131-123155, which deviate from the SIE+shear model prediction?
• RQ2 To what extent do intrinsic quasar variability and microlensing contribute to the observed flux variations in the lensed images over a period exceeding one year?
• RQ3 Can chromatic flux ratio variations be explained by differential extinction or microlensing effects?
• RQ4 Is the observed flux ratio discrepancy best explained by massive substructures (e.g., millilensing) or by higher-order multipole terms in the macro-lens potential?
• RQ5 What is the photometric accuracy achievable for point-like images when the Einstein ring is superimposed on the lensed quasar images?

Key findings

• The MCS deconvolution method enabled the first accurate relative and absolute photometry of the lensed quasar images from optical to near-infrared, with photometric accuracy approaching the photon noise limit in B and V bands.
• Flux variations of up to 0.3 mag were detected in image A, indicating significant intrinsic quasar variability combined with microlensing effects.
• Chromatic flux ratio variations suggest low differential extinction, supporting the presence of microlensing rather than dust effects.
• Microlensing alone cannot fully explain the anomalous flux ratios; under scenario S1 (de-amplification of saddle-point image A), microlensing is consistent with observations, but under S2 (simultaneous amplification of B and C), a massive substructure would be required to amplify A, a low-probability scenario.
• The observed flux ratios are poorly reproduced by the SIE+shear model, and millilensing from substructures cannot adequately explain the data, especially under S1.
• An octupole term (m=4) in the lens potential, introduced in Paper II, successfully reproduces both the observed flux ratio $I_{\rm{B}}/I_{\rm{C}}$ and the relative image positions, suggesting that complex macro-lens potentials may resolve the anomaly without invoking substructures.