Quasi-periodic scintillation patterns of the pulsars PSR 1133+16 and PSR 1642-03

Abstract
A new series of interstellar scintillation measurements of the pulsars PSR 1133+16 and PSR 1642−03 at 408 and 1426MHz made in 1982 and 1983 is presented. Quasi-periodic patterns are seen in the dynamic spectra of pulsar scintillation which change on time-scales ranging from one day to several months, but the coherence of these periodic modulations is maintained only over small portions of the time–frequency plane. These effects can be explained in terms of a theoretical model combining random diffraction from a number of small-scale electron density fluctuations in the interstellar medium with systematic refraction caused by larger irregularities. It is shown that when scintillation patterns are periodically modulated, the power-law index of the wavenumber spectrum of electron density irregularities measured in the range ∼1011−∼1013 cm exceeds a value of 4.

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