Theory and Simulations of Refractive Substructure in Resolved Scatter-Broadened Images

TL;DR

This paper investigates how interstellar scattering causes persistent refractive substructure in radio images, especially for unresolved sources, and discusses implications for high-resolution radio astronomy missions.

Contribution

It demonstrates that refractive substructure persists in extended sources and provides efficient methods for simulating realistic scattered images.

Findings

Refractive substructure can create spurious compact features in images.

Extended sources suppress but do not eliminate refractive substructure.

Simulations illustrate the impact of scattering on high-resolution observations.

Abstract

At radio wavelengths, scattering in the interstellar medium distorts the appearance of astronomical sources. Averaged over a scattering ensemble, the result is a blurred image of the source. However, Narayan & Goodman (1989) and Goodman & Narayan (1989) showed that for an incomplete average, scattering introduces refractive substructure in the image of a point source that is both persistent and wideband. We show that this substructure is quenched but not smoothed by an extended source. As a result, when the scatter-broadening is comparable to or exceeds the unscattered source size, the scattering can introduce spurious compact features into images. In addition, we derive efficient strategies to numerically compute realistic scattered images, and we present characteristic examples from simulations. Our results show that refractive substructure is an important consideration for ongoing missions at the highest angular resolutions, and we discuss specific implications for RadioAstron and the Event Horizon Telescope.