Meter-wavelength VLBI. IV - Temporal and spatial scattering of the Crab Nebula pulsar's radiation

Abstract

The spatial and temporal phenomena observed in the Crab Nebula pulsar's radiation are reviewed, with emphasis on the recent VLBI observations of the apparent angular size and the ratio of pulsing to total power. Three models for scattering (single thin screen, two screen, and extended medium) are discussed and shown to be deficient, under the usual assumption of a uniform scattering medium, in explaining the observed combination of temporal and angular broadening. A new model is proposed, in which the temporal broadening is dominated by scattering in the small filament which lies in the nebula in front of the pulsar, while the extended interstellar medium causes the angular broadening. This model explains the otherwise discordant angular and temporal effects and also the lack of an apparent source-size expansion with pulse phase.