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

Abstract

We use 13 seasons of R-band photometry from the 1.2 m Leonard Euler Swiss Telescope at La Silla to examine microlensing variability in the quadruply imaged lensed quasar WFI 2026-4536. The lightcurves exhibit ∼0.2 mag of uncorrelated variability across all epochs and a prominent single feature of ∼0.1 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°, we find an accretion disk scale radius of $\mathrm{log}({r}_{s}/\mathrm{cm})={15.74}_{-0.29}^{+0.34}$ at a rest-frame wavelength of 2043 Å, and we estimate a black hole mass of $\mathrm{log}({M}_{\mathrm{BH}}/{M}_{\odot })={9.18}_{-0.34}^{+0.39}$ , based on the C IV 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.