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

Observations of the effect of microlensing in gravitationally lensed quasars can be used to study the structure of active galactic nuclei on distance scales down to the sizes of the supermassive black holes powering source activity. We search for the microlensing in the signal from a gravitationally lensed blazar B0218+357 in very-high-energy gamma-ray band. We combine observations of a bright flare of the source in 2014 with Fermi/LAT and MAGIC telescopes in 0.1-100 GeV energy range. Using the time-delayed leading and trailing signals from two gravitationally lensed images of the source, we measure magnification factor at the moment of the flare. We use the scaling of the maximal magnification factor with the source size to constrain the size of gamma-ray emission region in the entire 0.1-100 GeV energy range. The magnification factor in the very-high-energy band derived from a comparison of Fermi/LAT and MAGIC data is $\mu_{VHE} = 36^{+40}_{-26}$, substantially larger than that in the radio band. This suggests that one of the source images is strongly affected by microlensing at the moment of the flare. Assuming that the microlensing is produced by a stellar mass object in the lens galaxy, we constrain 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 the spectrum of the microlensed source was unusually hard at the moment of the flare and speculate that this hardening may be due to the energy dependent microlensing effect. This interpretation suggests that the source size decreases with energy in 0.1-100 GeV energy range studied.