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Paper: Lenses Underwater: Scintillation from Very Small-Scale HI
Volume: 365, SINS — Small Ionized and Neutral Structures in the Diffuse Interstellar Medium
Page: 90
Authors: Gwinn, C.R.; Reynolds, J.E.; Wilson, W.W.
Abstract: Lenses become weaker when immersed in water. Similarly, interstellar scintillation changes when HI has small-scale structure. Lenses at optical wavelengths, and variations in electron density at radio wavelengths, introduce differences among path lengths. These paths of different lengths interfere to modulate intensity in the observer plane. The modulations represent focusing or defocusing for a lens, or interstellar scintillation for fluctuations in electron density. Interstellar atomic hydrogen changes path lengths, and opacity, near the 21-cm transition. If the 21-cm opacity or refractive index changes across the scattering disk, then the scintillation pattern in the observer plane will change rapidly with frequency, near the 21-cm line.

The Kramers-Kronig relations relate the maximum change in optical path length to the optical depth near the center of the absorption line. If the Doppler velocity of the 21-cm line shifts by a linewidth across the scattering disk, then a pulsar in strong scattering will change its scintillation pattern by about ΔI/I ≈ 2τ across the line. Smaller-scale variations in Doppler velocity can yield even stronger intensity variations. Here, we apply these ideas to observations of HI absorption toward a scintillating pulsar.

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