A Microlensing Accretion Disk Size Measurement in the Lensed Quasar WFI 2026-4536

TL;DR

This study measures the size of a quasar's accretion disk using microlensing variability over thirteen seasons, finding it larger than thin disk theory predicts and consistent with the black hole mass relation.

Contribution

It provides a new microlensing-based measurement of an accretion disk size in a quadruply-imaged quasar, confirming the disk is larger than thin disk models suggest.

Findings

Accretion disk scale radius: log(r_s/cm)=15.74^{+0.34}_{-0.29}

Black hole mass: log(M_BH/M_sun)=9.18^{+0.39}_{-0.34}

Disk size exceeds thin disk predictions

Abstract

We use thirteen seasons of R-band photometry from the 1.2m Leonard Euler Swiss Telescope at La Silla to examine microlensing variability in the quadruply-imaged lensed quasar WFI 2026-4536. The lightcurves exhibit ${\sim}\,0.2\,\text{mag}$ of uncorrelated variability across all epochs and a prominent single feature of ${\sim}\,0.1\,\text{mag}$ within a single season. We analyze this variability to constrain the size of the quasar's accretion disk. Adopting a nominal inclination of 60$^\text{o}$, we find an accretion disk scale radius of $\log(r_s/\text{cm}) = 15.74^{+0.34}_{-0.29}$ at a rest-frame wavelength of $2043\,\unicode{xC5}$, and we estimate a black hole mass of $\log(M_{\text{BH}}/M_{\odot}) = 9.18^{+0.39}_{-0.34}$, based on the CIV line in VLT spectra. This size measurement is fully consistent with the Quasar Accretion Disk Size - Black Hole Mass relation, providing another system in which the accretion disk is larger than predicted by thin disk theory.