Multi-Epoch VLBI observations of the extreme scintillator J1819+3845

The quasar J1819+3845 has shown extreme variability with flux density variations in the radio regime up to 600% in less than one hour. In case of intrinsic high varibility, the short time scale sets a limit on the size of the emitting region and allows to estimate its brightness temperature. This would exceed 10²¹ K in the case of J1819+3845. Even an high relativistic jet beamed and doppler boosted in our line of sight cannot explain such an extreme violation of the Inverse Compton limit (10¹² K). The variability of this source has been proven to be due to scattering in the Interstellar medium by a number of different experiments. Such an explanation requires a closeby scattering screen (few parsecs) and it results in a brightness temperature of about 10¹⁴ K. Many observing campaigns have been carried on to map the innermost jet structures of J1819+3845. Here we present the results of a number of VLBI observations, including space VLBI, to search for the missing jet in this puzzling source.