Extremely anisotropic scintillations

A small number of quasars exhibit interstellar scintillation on time-scales less than an hour; their scintillation patterns are all known to be anisotropic. Here, we consider a totally anisotropic model in which the scintillation pattern is effectively one-dimensional. For the persistent rapid scintillator J1819+3845, we show that this model offers a good description of the two-station time-delay measurements and the annual cycle in the scintillation time-scale. Generalizing the model to finite anisotropy yields a better match to the data but the improvement is not significant and the two additional parameters which are required to describe this model are not justified by the existing data. In contrast, our data for the persistent rapid scintillator PKS1257-326 are significantly better fit by a two-dimensional model with a major-to-minor axis ratio of 6 for the scintillation pattern. For J1819+3845, the totally anisotropic model predicts that the particular radio flux variations seen between mid-July and late August should repeat between late August and mid-November, and then again between mid-November and late December as the Earth twice changes its direction of motion across the scintillation pattern. If this effect can be observed then the minor-axis velocity component of the screen and the orientation of that axis can both be precisely determined. In reality, the axis ratio is finite, albeit large and spatial decorrelation of the flux pattern along the major axis may be observable via differences in the pairwise fluxes within this overlap region; in this case, we can also constrain both the major-axis velocity component of the screen and the magnitude of the anisotropy.