Constraints on the size of the emitting region in an active galactic nucleus

The microlensing phenomenon observed in the quasar $${\rm Q}\, 2237 + 0305$$ strongly constrains the size of the region emitting in the optical band. It rules out a model of optical/UV emission produced in a single optically thin cloud. Both a model with a huge number of small optically thin cloudlets and a model with an accretion disc around a supermassive black hole are, however, marginally consistent with the data. An irradiated disc model can both meet the size requirements and reproduce the slope of the quasar spectrum, even if the effects of electron scattering in the atmosphere of the disc are taken into account. Future observations of colour changes in $${\rm Q}\, 2237 + 0305$$ are crucial to distinguish these two possibilities.