Modeling of Interstellar Scintillation Arcs from Pulsar B1133+16

The parabolic arc phenomenon visible in the Fourier analysis of the scintillation spectra of pulsars provides a new method of investigating the small-scale structure in the ionized interstellar medium (ISM). We report archival observations of the pulsar B1133+16 showing both forward and reverse parabolic arcs sampled over 14 months. These features can be understood as the mutual interference between an assembly of discrete features in the scattered brightness distribution. By model fitting to the observed arcs at one epoch, we obtain a "snap-shot" estimate of the scattered brightness, which we show to be highly anisotropic (axial ratio >10 : 1), to be centered significantly off axis, and to have a small number of discrete maxima, which are coarser than the speckle expected from a Kolmogorov spectrum of interstellar plasma density. The results suggest that these are the effects of highly localized discrete scattering regions that subtend 0.1-1 mas but can scatter (or refract) the radiation by angles that are 5 or more times larger.