Evidence for Microarcsecond Structure in the BL Lacertae Object AO 0235+164

Abstract

We have been investigating short timescale variability phenomena in some blazars with the Westerbork Synthesis Radio Telescope (WSRT). Low-modulation intrahour variability is detected at 6 cm in the BL Lac object AO 0235+164. If such a variation were intrinsic to the source, it would imply an apparent brightness temperature of ~1019 K in the source comoving frame, requiring a Doppler factor of ~364 to reconcile it with the 1012 K inverse Compton limit. A gravitational microlensing origin seems unlikely. We conclude that the variations are best explained by interstellar scintillation and show that the source must be larger than 15 μas. This implies an apparent brightness temperature lower than 1013 K and can be interpreted by a relativistic component moving with a Doppler factor smaller than 38. With the scintillation model we estimate that the scattering occurs in material at less than 60 pc from the Earth.