Anomalous Radio‐Wave Scattering from Interstellar Plasma Structures

This paper considers scattering screens that have arbitrary spatialvariations of scattering strength transverse to the line of sight, includingscreens that are spatially well confined, such as disks and filaments. Wecalculate the scattered image of a point source and the observed pulse shape ofa scattered impulse. The consequences of screen confinement include: (1) Sourceimage shapes that are determined by the physical extent of the screen ratherthan by the shapes of much-smaller diffracting microirregularities. Theseinclude image elongations and orientations that are frequency dependent. (2)Variation with frequency of angular broadening that is much weaker than thetrademark \nu^{-2} scaling law (for a cold, unmagnetized plasma), includingfrequency-independent cases; and (3) Similar departure of the pulse broadeningtime from the usually expected \nu^{-4} scaling law. We briefly discussapplications that include scattering of pulses from the Crab pulsar byfilaments in the Crab Nebula; image asymmetries from Galactic scattering of thesources Cyg X-3, Sgr A*, and NGC 6334B; and scattering of background activegalactic nuclei by intervening galaxies. We also address the consequences forinferences about the shape of the wavenumber spectrum of electron densityirregularities, which depend on scaling laws for the image size and the pulsebroadening. Future low-frequency (< 100 MHz) array observations will also bestrongly affected by the Galactic structure of scattering material. Ourformalism is derived in the context of radio scattering by plasma densityfluctuations. It is also applicable to optical, UV and X-ray scattering bygrains in the interstellar medium.Comment: 21 pages, LaTeX2e with AASTeX-4.0, 6 PostScript figures, accepted by ApJ, revised version has minor changes to respond to referee comments and suggestion