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

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 similar to 0.2 mag of uncorrelated variability across all epochs and a prominent single feature of similar to 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 degrees, we find an accretion disk scale radius of (r(s)/cm)= 15.74(-0.29)(+0.34) at a rest frame wavelength of 2043 angstrom, and we estimate a black hole mass of log (M-BH/M-circle dot) 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.