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

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 $\log (r_s\mathrm{/}\text{cm})=15.74_{-0.29}^{+0.34}$ at a rest-frame wavelength of $2043\text{unicode}xC5$, and we estimate a black hole mass of $\log (M_{\text{BH}}\mathrm{/}{M}_{\odot })=9.18_{-0.34}^{+0.39}$, 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.