Measurement of the size of the gamma-ray source in blazar B0218+357 based on microlensing at the 100 GeV energy band

Abstract

Context. 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 a supermassive black hole’s powering source activity. Aims. We searched for a microlensing effect in the signal from a gravitationally lensed blazar, B0218+357, in a very-high-energy γ-ray band. Methods. We combined observations of a bright flare of the source in 2014 by the Fermi Large Area Telescope and MAGIC telescopes in the 0.1 − 300 GeV and 65 − 175 GeV energy ranges, respectively. Using the time-delayed leading and trailing signals from two gravitationally lensed images of the source, we measured the magnification factor at the moment of the flare. We used the scaling of the maximal magnification factor with the source size to constrain the size of the γ-ray emission region in the wide 0.1 − 175 GeV energy range. Results. The magnification factor in the very-high-energy band that we derived from our comparison of Fermi/LAT and MAGIC data is μVHE = 25−17+38, which is substantially larger than the factor found in the radio band. This suggests 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 constrained the size of the emission region in the E > 100 GeV band to be RVHE = 6.6−5.6+69 × 1014 cm. We note that the spectrum of the microlensed source was unusually hard at the moment of the flare, and we speculate that this hardening may be due to the energy-dependent microlensing effect. This interpretation suggests that the source size decreases with energy in the entire 0.1 − 175 GeV energy range we studied.