Determining the innermost structure of quasars by microlensing: Measuring, simulating and interpreting light curves of multiple quasars

This thesis aims at measuring quasar microlensing light curves and applying them to constrain the structure of quasar accretion disks. Thus, data was taken in two photometric filters at the Las Cumbres Observatory, using their global network of 1m telescopes since 2014. In the first part, applying difference imaging analysis together with point spread function photometry aided with Gaia data, we measure the light curves of the multiple images of eight gravitationally lensed quasars in the R and V band, covering almost ten years with in total 1872 epochs. For each quasar, we determine difference curves of the time delay corrected light curves. This removes the intrinsic quasar brightness variations present in all images with only uncorrelated microlensing variability of the individual images remaining. We find these additional variations, attributed to the source size depended microlensing of the individual images by compact objects in the lens galaxy, throughout our whole data set. For the second part of this thesis, we focus on the prominent microlensing signal in image B of the quadruple quasar HE0435-1223, revealed through our difference curves. The variations appear to be chromatic, i. e. depend on the filter, with higher amplitude fluctuations in the V band. This is expected, since the hotter central region of the accretion disk experiences more microlensing variation due to its smaller size. To quantify this observation, by means of microlensing simulations, we are able to infer that the accretion disk of HE0435-1223 is indeed larger in radius by factors of 1. 24^+0. 08−₀. ₂₀, 1. 42^+0. 11−₀. ₂₂ and 1. 43^+0. 10−₀. ₂₃ in the R with respect to the V band, depending on the disk model, in agreement with the expectation from thin accretion disk theory, though with a tendency towards a shallower temperature profile. Additionally, we find disk half-light radii of 0. 7 to 1. 0 Einstein radii, corresponding to average inclined disk scale radii of around log ⟨R2500/cm⟩ ≃ 16. 4^+0. 5−₀. ₇ at 2500Å in the quasar rest-frame.