Worst-case Microlensing in the Identification and Modeling of Lensed Quasars

Although microlensing of macrolensed quasars and supernovae provides unique opportunities for several kinds of investigations, it can add unwanted and sometimes substantial noise. While microlensing flux anomalies may be safely ignored for some observations, they severely limit others. Worst-case estimates can inform the decision whether or not to undertake an extensive examination of microlensing scenarios. Here, we report worst-case microlensing uncertainties for point sources lensed by singular isothermal potentials, parameterized by a convergence equal to the shear and by the stellar fraction. The results can be straightforwardly applied to nonisothermal potentials by utilizing the mass sheet degeneracy. We use microlensing maps to compute the fluctuations in image micromagnifications and estimate the stellar fraction at which the fluctuations are greatest for a given convergence. We find that the worst-case fluctuations happen at the stellar fraction kappa=1 divide mu macro divide . For macrominima, the fluctuations in both magnification and demagnification appear to be bounded (1.5 > Delta m > -1.3, where Delta m is the magnitude relative to the average macromagnification). Magnifications for macrosaddles are bounded as well (Delta m > -1.7). In contrast, demagnifications for macrosaddles appear to have unbounded fluctuations as 1/mu (macro) -> 0 and kappa -> 0.

Automated summary

Although microlensing studies offer unique opportunities, their uncertainties and fluctuations can introduce noise to observations. Worst-case estimates can help in deciding whether a thorough examination of microlensing scenarios is necessary.