The discovery of a microarcsecond quasar: J1819+3845

We report on the discovery of a source that exhibits over 300% amplitude ch anges in radio flux density on the period of hours. This source, J1819+3845 , is the most extremely variable extragalactic source known in the radio sk y. We believe these properties are due to interstellar scintillation and sh ow that the source must emit at least 55% of its flux density within a radi us of fewer than 16 mu as at 5 GHz. The apparent brightness temperature is greater than 5 x 10(12) K, and the source may be explained by a relativisti cally moving source with a Doppler factor of similar to 15. The scattering occurs predominantly in material only a few tens of parsecs from the Earth, which explains its unusually rapid variability. If the source PKS 0405-385 is similarly affected by local scattering material, Doppler factors of sim ilar to 1000 are not required to explain this source. The discovery of a se cond source whose properties are well modeled by interstellar scintillation strengthens the argument for this as the cause for much of the variation s een in intraday variables.