Scintillations and Levy flights through the interstellar medium

Abstract

Temporal broadening of pulsar signals results from electron density fluctuations in the interstellar medium that cause the radiation to travel along paths of different lengths. The Gaussian theory of fluctuations predicts that the pulse temporal broadening should scale with the wavelength as lambda^4, and with the dispersion measure (corresponding to distance to the pulsar) as DM^2. For large dispersion measure, DM > 20 pc/cm^3, the observed scaling is lambda^4 DM^4, contradicting the conventional theory. Although the problem has existed for 30 years, there has been no resolution to this paradox. We suggest that scintillations for distant pulsars are caused by non-Gaussian, spatially intermittent density fluctuations with a power-like probability distribution. This probability distribution does not have a second moment in a large range of density fluctuations, and therefore the previously applied conventional Fokker-Planck theory does not hold. Instead, we propose to apply the theory of Levy distributions (so-called Levy flights). Using the scaling analysis (confirmed by numerical simulations of ray propagation) we show that the observed scaling is recovered for large DM, if the density differences, delta N, have Levy distribution decaying as |delta N|^{-5/3}.