Improved Constraints on the Gravitational Lens Q0957+561. I. Weak Lensing

TL;DR

This study uses weak gravitational lensing data from Hubble Space Telescope imaging to measure the average mass sheet around the lens galaxy in Q0957+561, aiming to reduce systematic uncertainties in Hubble constant estimates.

Contribution

The paper introduces a weak-lensing analysis to constrain the mass sheet degeneracy in the gravitational lens system Q0957+561, providing new measurements to improve lens modeling accuracy.

Findings

Average convergence within 30" radius is 0.166±0.056.

Constraints on low-order lens potential terms from shear data.

Implications for reducing systematic uncertainties in H0 estimates.

Abstract

Attempts to constrain the Hubble constant using the strong gravitational lens system Q0957+561 are limited by systematic uncertainties in the mass model, since the time delay is known very precisely. One important systematic effect is the mass sheet degeneracy, which arises because strong lens modeling cannot constrain the presence or absence of a uniform mass sheet $\kappa$, which rescales $H_0$ by the factor $(1-\kappa)$. In this paper we present new constraints on the mass sheet derived from a weak-lensing analysis of the Hubble Space Telescope imaging of a 6 arcmin square region surrounding the lensed quasar. The average mass sheet within a circular aperture (the strong lens model region) is constrained by integrating the tangential weak gravitational shear over the surrounding area. We find the average convergence within a $30"$ radius around the lens galaxy to be $\kappa(<30") = 0.166\pm0.056$ ($1\sigma$ confidence level), normalized to the quasar redshift. This includes contributions from both the lens galaxy and the surrounding cluster. We also constrain a few other low-order terms in the lens potential by applying a multipole aperture mass formalism to the gravitational shear in an annulus around the strong lensing region. Implications for strong lens models and the Hubble constant are discussed in an accompanying paper.