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Observations of the effect of microlensing in gravitationally lensed quasarscan be used to study the structure of active galactic nuclei on distance scalesdown to the sizes of the supermassive black holes powering source activity. Wesearch for the microlensing in the signal from a gravitationally lensed blazarB0218+357 in very-high-energy gamma-ray band. We combine observations of abright flare of the source in 2014 with Fermi/LAT and MAGIC telescopes in0.1-100 GeV energy range. Using the time-delayed leading and trailing signalsfrom two gravitationally lensed images of the source, we measure magnificationfactor at the moment of the flare. We use the scaling of the maximalmagnification factor with the source size to constrain the size of gamma-rayemission region in the entire 0.1-100 GeV energy range. The magnificationfactor in the very-high-energy band derived from a comparison of Fermi/LAT andMAGIC data is $\mu_{VHE} = 36^{+40}_{-26}$, substantially larger than that inthe radio band. This suggests that one of the source images is stronglyaffected by microlensing at the moment of the flare. Assuming that themicrolensing is produced by a stellar mass object in the lens galaxy, weconstrain the size of the emission region in the $E>100$ GeV band to be$\mathrm{R_{VHE} = 2.2^{+27}_{-1.7} \times 10^{13}~cm}$. We note that thespectrum of the microlensed source was unusually hard at the moment of theflare and speculate that this hardening may be due to the energy dependentmicrolensing effect. This interpretation suggests that the source sizedecreases with energy in 0.1-100 GeV energy range studied.

Measurement of size of gamma-ray source in blazar B0218+357 from microlensing at 100 GeV energy