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Session 15 - Interstellar Medium I.
Display session, Monday, January 15
North Banquet Hall, Convention Center
Discrete plasma irregularities (plasma lenses) in the interstellar medium will strongly refract the ray paths of background radio sources. As the lens moves across the line of sight from observer to background source, refractive effects will include strong modulations in the radio light curves of the source, displacement of the source's apparent position, and possibly multiple imaging of the source. Interstellar plasma lenses may be the cause of several interesting phenomena, including extreme scattering events (ESE's, Fiedler et al. 1987, Nature, 326, 675) and discontinuities in the dynamic spectra of pulsars (Clegg, Fiedler, amp; Cordes 1993, ApJ, 409, 691). The refractive properties of interstellar plasma lenses have been examined in detail (Clegg, Fey, amp; Fiedler, in preparation). In that work, we assumed a simple functional form for the lens (a Gaussian distribution of electron density with a specified width).
The lenses will also impart diffractive effects, including distortions in the image of the background source and coherent interference between multiple images. In addition, the amplitudes of caustics in the radio light curves of ESE sources can only be accurately predicted by including the effects of diffraction in lens simulations. Motivated by the availability of a large number of light curves of extragalactic sources undergoing ESE's, at least one high angular resolution VLBI observation of an extragalactic source (1741-038) during an ESE, and the existence of a large database of pulsar dynamic spectra exhibiting strong lensing phenomena, we have begun to examine in detail the diffractive properties of interstellar Gaussian plasma lenses. This work complements the refractive simulations previously undertaken, and will allow additional information on the physical characteristics of the lensing medium to be ascertained.
Initial results of our diffraction simulations will be presented.
