Twinkle, twinkle little pulsar/quasar

The twinkling of stars is a familiar example of scintillations, due to turbulence in the Earth’s atmosphere causing fluctuations in the refractive index of the air along the line of sight. Scintillations lead to time variations in the apparent position of the source, and hence to an angular broadening on integration over an observation time. Scintillations also lead to fluctuations in the intensity of the source. Pointlike astronomical radio sources such as pulsars and (the compact cores of some) quasars scintillate due to fluctuations in the electron density along the line of sight through the interstellar medium. For quasars, low-frequency (100s of MHz) variability over periods of years is a scintillation effect, as are probably more rapid (as short as an hour) intensity variations at higher radio frequencies. Unlike the twinkling of stars, which is due to weak scintillations, the scintillations of radio sources are usually strong. Important qualitative effects associated with strong scattering are multipath propagation and a clear separation into diffractive and refractive scintillations. Quasars exhibit only refractive scintillations. Pulsars are extremely small and bright, and they vary temporally on a very short time scale, making them almost ideal sources on which to test our ideas on scintillations. Pulsars exhibit a variety of scintillation phenomena, due to both refractive and diffractive effects, the latter seen most clearly in dynamic spectra. These data are used to model the distribution of electrons through the Galaxy, to determine the distribution of pulsar velocities, and potentially to resolve the source region in a pulsar magnetosphere. These scintillation phenomena and their interpretation in terms of the theory of strong scintillations are reviewed briefly. The generalisation of the theory to include the birefringence of the plasma (Faraday effect), and its possible implications on the interpretation of circular polarisation, are then outlined. An attempt to generalise the theory to describe scattering by a distribution of discrete scattering objects is also discussed briefly.