The effects of caustics on scintillating radio sources

• Published in: Monthly notices of the Royal Astronomical Society 1987-11, Vol.229 (1), p.73-102
• Main Authors: Goodman, Jeremy J., Romani, Roger W., Blandford, Roger D., Narayan, Ramesh
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Radio sources; Stellar systems. Galactic and extragalactic objects and systems. The universe; Unidentified sources and radiation outside the solar system
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/229.1.73

We consider the scintillation properties of compact radio sources 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, we find 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 of 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. Caustics are best observed by monitoring pulsar fluxes through a range of frequencies and specific observations are suggested which should either detect their effects or delimit their importance. We conclude that caustic effects are only likely to dominate the flux variations when the inner scale is ∼10 per cent of the refractive scale. For somewhat larger inner scales individual caustics could be resolved. Whether such conditions obtain in the ISM is still uncertain; if they do not, a power-spectrum description may not be adequate and focusing events from single scatterers (i.e. rare high-density clouds instead of an incoherent superposition of small-amplitude density fluctuations along the line-of-sight) may dominate the refractive variations.