Measuring the size of quasar broad-line clouds through time-delay light-curve anomalies of gravitational lenses

Intensive monitoring campaigns have recently attempted to measure the time delays between multiple images of gravitational lenses. Some of the resulting light curves show puzzling low-level, rapid variability that is unique to individual images, superposed on (and concurrent with) longer timescale intrinsic quasar variations that repeat in all images. We demonstrate that both the amplitude and variability timescale of the rapid light-curve anomalies are naturally explained by stellar microlensing of a smooth accretion disk that is occulted by optically thick broad-line clouds. This model also explains the observed correlation between microlensing and intrinsic variability. The rapid timescale is caused by the high velocities of the clouds (∼5 × 10 ³ km s^-1 ), and the low-amplitude results from the large number of clouds covering the magnified or demagnified parts of the disk. The observed amplitudes of variations in specific lenses implies that the number of broad-line clouds that cover ∼10% of the quasar sky is ∼10⁵ per 4π sr. This is comparable to the expected number of broad-line clouds in models where the clouds originate from bloated stars.