Revealing the structure of the lensed quasar Q 0957+561: I. Accretion disk size

TL;DR

This study uses long-term photometric data and microlensing analysis to estimate the size of the accretion disk in the gravitationally lensed quasar Q 0957+561, revealing an unusually large optical disk.

Contribution

It introduces a robust Bayesian method to determine accretion disk size from microlensing signatures using extensive 21-year light curves.

Findings

Accretion disk half-light radius is approximately 17.6 lt-days.

The quasar's optical disk is unusually large compared to typical sizes.

The method effectively accounts for uncertainties in microlensing-based size estimates.

Abstract

We aim to use signatures of microlensing induced by stars in the foreground lens galaxy to infer the size of the accretion disk in the gravitationally lensed quasar Q 0957+561. The long-term photometric monitoring of this system (which so far has provided the longest available light curves of a gravitational lens system) permits us to evaluate the impact of uncertainties on our recently developed method (controlled by the distance between the modeled and the experimental magnitude difference histograms between two lensed images), and thus to test the robustness of microlensing-based disk-size estimates. We analyzed the well-sampled 21-year GLENDAMA optical light curves of the double-lensed quasar and studied the intrinsic and extrinsic continuum variations. Using accurate measurements for the time delay between the images A and B, we modeled and removed the intrinsic quasar variability, and from the statistics of microlensing magnifications we used a Bayesian method to derive the size of the region emitting the continuum at 2558 angstroms. Analyses of the Q 0957+561 R-band light curves show a slow but systematic increase in the brightness of the B relative to the A component during the past ten years. The relatively low strength of the magnitude differences between the images indicates that the quasar has an unusually big optical accretion disk of half-light radius $R_{1/2} = 17.6 \pm 6.1 \sqrt{M/0.3M_\odot}$ lt-days.