Quasar microlensing offers a unique opportunity to resolve tiny sources in distant active galactic nuclei and study compact object populations in lensing galaxies .
We therefore searched for microlensing-induced variability of the gravitationally lensed quasar QSO 2237+0305 ( Einstein Cross ) using 4 374 optical frames taken with the 2.0 m Liverpool Telescope and the 1.5 m Maidanak Telescope .
These gVrRI frames over the 2006 - 2019 period were homogeneously processed to generate accurate long-term multi-band light curves of the four quasar images A-D .
Through difference light curves , we found strong microlensing signatures .
We then focused on the analytical modelling of two putative caustic-crossing events in image C , finding compelling evidence that this image experienced a double caustic crossing .
Additionally , our overall results indicate that a standard accretion disc accounts reasonably well for the brightness profile of UV continuum emission sources and for the growth in source radius when the emission wavelength increases : R _ { \lambda } \propto \lambda ^ { \alpha } , \alpha = 1.33 \pm 0.09 .
However , we caution that numerical microlensing simulations are required before firm conclusions can be reached on the UV emission scenario because the VRI -band monitoring during the first caustic crossing and one of our two \alpha indicators lead to a few good solutions with \alpha \approx 1 .