Low-frequency variability of pulsars

Several recent observational and theoretical investigations have suggested that long-wavelength ($$\lesssim10^{14} \enspace \text {cm}$$) density fluctuations in the interstellar medium may have a major influence on observations of pulsars and extragalactic radio sources. These long-wavelength fluctuations are responsible for refractive focusing (in contrast to the diffractive scintillation which is conventionally attributed to shorter wavelength fluctuations), and may cause the monthly to annual variations in pulsar flux densities observed at low frequency. Fluctuations in the angular sizes, pulse arrival times and pulse widths of pulsars should also be observable and should be cross-correlated with the flux density variations. The magnitude of these correlations, and their dependence upon the time lag and the observing wavelength, for different power-law spectra of density fluctuations are estimated. It appears that if the flux variability is interstellar in origin, then the spectrum must have a somewhat steeper logarithmic slope that the value (∼11/3) given by the standard ‘Kolmogorov’ spectrum. Sensitive observations on selected pulsars will be able to confirm the importance of refractive effects in the interstellar medium and also determine the slope of the density fluctuation spectrum.