The effects of caustics on scintillating radio sources

The scintillation properties of compact radio sources are considered for a spectrum of interstellar electron density fluctuations which is a power law over a finite range of spatial frequencies. In particular, if the power law is truncated at an inner scale intermediate between the diffractive scale and the refractive scale, it is found that there is additional power in the spectrum of the intensity variations at these scales. This power is associated with strong focusing events, or caustics. These events are best described as simple diffraction catastrophes, which are classified and analysed on the basis of geometrical and wave optics, taking account the strong dispersion of the ISM scattering, which introduces important frequency dependencies. It is shown that such caustics can enhance the flux modulation and may produce patterns in dynamic spectra similar to those observed. (author)