Microarcsecond Resolution with Interstellar Scintillation

Abstract

The rapid, intra- and inter-day variability now seen at cm wavelengths in many compact, flatspectrum radio sources, was discovered almost thirty years ago based on accurate flux density measurements made with the Effelsberg 100 m radio telescope. It was initially thought to be intrinsic to the sources themselves. However, accumulated evidence now strongly favours interstellar scintillation, ISS, in the turbulent, interstellar medium of our Galaxy, as the principal mechanism responsible for such rapid variability. For a source to exhibit ISS it must contain a compact component whose angular size is comparable to the angular size of the first Fresnel zone; for reasonable screen distances this implies microarcsecond component sizes. ISS now makes it possible to probe source structure with microarcsecond resolution, finer than that achieved with ground-based VLBI and the equal of that achievable now on the longest space baselines with RadioAstron. The presence of ISS therefore has significant implications for VLBI astronomy, astrometry and geodesy