Microlensing as a Delay Contaminant: Which Systems Have Their Cosmography Taxed by Stars

Stars in the lens galaxy microlens the quasar's accretion disc, and because the disc is not a point, microlensing shifts the measured time delay of each image by an amount that varies with the microlensing configuration. Adopting perturbation amplitudes of half a day to two days, representative of published calculations, the added fractional delay error is tabulated across delay lengths. The worst case computed, a two-day perturbation on a ten-day delay, adds 20.0 per cent; on a hundred-day delay the same perturbation adds two per cent. Short-delay systems are therefore taxed at a rate that can exceed their entire modelling budget, while long-delay systems are nearly immune. Delay programmes should carry a microlensing line item scaled by the tabulated fractions rather than treating light-curve scatter as pure measurement noise. Computed under stated amplitudes; not measured.