CAOSP abstracts, Volume: 56, No.: 1, year: 2026

Abstract: We present a novel method to derive rotation curves of the inner broad-line region (BLR) of lensed quasars with light-day spatial resolution. The approach exploits microlensing distortions of the broad emission lines (BELs), where the strength of the effect in the line wings traces the size of the emitting region at different velocities. We analyze the high-ionization lines Si IV and C~IV in five gravitationally lensed quasars, measuring microlensing amplitudes across several velocity bins. Bayesian inference yields emission-region sizes, which we confront with a Keplerian disk model. We find a smooth, monotonic increase in microlensing with velocity, and the derived velocity-size relations are consistent with disk-like rotation. These results provide the first direct evidence for Keplerian motion in the innermost BLR of quasars.