Central engine of a gamma-ray blazar resolved through the magnifying glass of gravitational microlensing

Abstract

Gamma-ray emission from blazars is known to originate from jets emitted by supermassive black holes. However, the exact location and size of the -ray emitting part of the jets is uncertain. The main difficulty is the very small angular size of these sources, beyond the angular resolution of -ray telescopes. Here, we report a measurement of the projected size of the -ray jet, revealed by the detection of microlensing in the gravitationally lensed blazar PKS 1830-211. This measurement is consistent with a constraint from the intrinsic variability timescale of the blazar. Our measurement shows that the -ray emission originates from the vicinity of the central supermassive black hole. Combining the X-ray and -ray data, we use the microlensing effect to constrain the size of the X-ray source. We show that the effect of pair production of -rays on X-ray photons does not make the source opaque, owing to the large size of the X-ray emission region.