Source Size Limitation from Variabilities of a Lensed Quasar

In the case of gravitationally-lensed quasars, it is well-known that there is a time delay between occurrence of the intrinsic variabilities in each split image. Generally, the source of variabilities has a finite size, and there are time delays even in one image. If the origin of variabilities is widely distributed, say over $\gsim 100 pc$ as whole, variabilities between split images will not show a good correlation even though their origin is identical. Using this fact, we are able to limit the whole source size of variabilities in a quasar below the limit of direct resolution by today's observational instruments.