Detection of six rapidly scintillating active galactic nuclei and the diminished variability of J1819+3845

The extreme, intra-hour and >10% rms flux density scintillation observed in AGNs such as PKS 0405-385, J1819+3845 and PKS 1257-326 at cm wavelengths has been attributed to scattering in highly turbulent, nearby regions in the interstellar medium. Such behavior has been found to be rare. We searched for rapid scintillators among 128 flat spectrum AGNs and analyzed their properties to determine the origin of such rapid and large amplitude radio scintillation. The sources were observed at the VLA at 4.9 and 8.4 GHz simultaneously at two hour intervals over 11 days. We detected six rapid scintillators with characteristic time-scales of <2 h, none of which have rms variations >10%. We found strong lines of evidence linking rapid scintillation to the presence of nearby scattering regions, estimated to be <12 pc away for similar to 200 mu as sources and <250 pc away for similar to 10 mu as sources. We attribute the scarcity of rapid and large amplitude scintillators to the requirement of additional constraints, including large source compact fractions. J1819+3845 was found to display similar to 2% rms variations at similar to 6 h time-scales superposed on longer >11 day variations, suggesting that the highly turbulent cloud responsible for its extreme scintillation has moved away, with its scintillation now caused by a more distant screen approximate to 50 to 150 pc away.