Fitting formula for flux scintillation of compact radio sources

We present a fitting function to describe the statistics of flux modulations caused by interstellar scintillation. The function models a very general quantity: the cross-correlation of the flux observed from a compact radio source of finite angular size observed at two frequencies and at two positions or times. The formula will be useful for fitting data from sources such as intraday variables and gamma-ray burst afterglows. These sources are often observed at relatively high frequencies (several gigahertz), where interstellar scattering is neither very strong nor very weak, so that asymptotic formulae are inapplicable.