Spectral characteristics of low-frequency variability in compact extragalactic radio sources
We examine the refractive scintillation hypothesis of low frequency variability via numerical simulation in order to account for the spectral characteristics of the observed fluctuations. Plane waves from extragalactic radio sources propagating through the interstellar medium, a medium with fluctuations of the refractive index due to electron density irregularities, emerge from that medium corrugated. Since fluctuations present on scales greater than the Fresnel scale act refractively, the emerging wavefront has a curvature which produces a refractive amplification or deamplification in the flux density. We develop a numerical algorithm to characterize the phase fluctuations in the wavefront and we simulate the resulting intensity distribution in the limit of geometrical optics. We then produce light curves by taking trajectories in the simulated intensity distribution plane and we compare our results with statistical properties of existing data from a 5—year monitoring program of 32 extragalactic sources at frequencies 0.318, 0.430, 0.606, 0.880 and 1.4 GHz. We find that the refractive scintillation hypothesis is in good agreement with the data at comparable timescales and that the variability in the simulated light curves diminishes within an octave of frequency, in agreement with those of the observed light curves with the same behavior.