Anomalous radio-wave scattering from interstellar plasma structures

This paper considers scattering screens that have arbitrary spatial variati ons of scattering strength transverse to the line of sight, including scree ns that are spatially well confined, such as disks and filaments. We calcul ate the scattered image of a point source and the observed pulse shape of a scattered impulse. The consequences of screen confinement include (1) sour ce image shapes that are determined by the physical extent of the screen ra ther than by the shapes of much smaller diffracting microirregularities (th ese include image elongations and orientations that are frequency dependent ); (2) variation with frequency of angular broadening that is much weaker t han the trademark nu (-2) scaling law (for a cold, unmagnetized plasma), in cluding frequency-independent cases; and (3) similar departure of the pulse -broadening time from the usually expected nu (-4) scaling law. We briefly discuss applications that include scattering of pulses from the Crab pulsar by filaments in the Crab Nebula; image asymmetries from Galactic scatterin g of the sources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of backgrou nd active galactic nuclei by intervening galaxies. We also address the cons equences for inferences about the shape of the wavenumber spectrum of elect ron density irregularities, which depend on scaling laws for the image size and the pulse broadening. Future low-frequency (<100 MHz) array observatio ns will also be strongly affected by the Galactic structure of scattering m aterial. Our formalism is derived in the context of radio scattering by pla sma density fluctuations. It is also applicable to optical, UV, and X-ray s cattering by grains in the interstellar medium.