Interstellar Scintillation of Pulsar Radiation

It is demonstrated that the long-term fluctuations in the intensity of pulsa radiation are due to interstellar scintillation. Observed statistical quantities are consistent with a thin-screen model in which the screen is located halfway between the Earth and the pulsar. Decorrelation frequencies are found to be proportional to the fourth power of the observing frequency and inversely proportional to the square of the dispersion measure. Power spectra are nearly Gaussian in form, but have a high- frequency tail which may be power law. Observed pulse broadening is shown to be equivalent to the convolution of the emitted pulse profile wfth an exponential function for which the decay time is the reciprocal of the decorrelation frequency. Decorrelation times are found to be proportional to the observing frequency and weakly dependent on dispersion measure. Iodulation indices are nearly unity and independent of observing frequency in the interval 111-606 MHx. Both Rice-squared and lognormal probability distribution functions fit the observed probability distributions of intensity well. The scale size is 1011 cm, roughly the size of the first Fresnel zone, and the rms fluctuating number density of electrons is 3 X l0- . The scattering becomes weak above 2000 f liz for nearby pulsars. These scintillation measurements provide estimates for pulsar distances which, when used wfth known dispersion measures, indicate that the average number density of electrons along the line of sight to the pulsars is 0.03 . Pulsars must be smaller than 10- seconds of arc in order to give rise to the observed intensity fluctuations. This size is considerably below the angular size of any observable self-absorbed synchrotron radio source. The relative velocities of the pulsar and/or the interstellar medium with respect to the Earth lie in the range 30-200 km . Pulsars with periods smaller than a few milliseconds will not be detected at frequencies below a few hundred Mlix. The minimum detectable angular size of a radio source at a distance of 300 pc is on the order of 10-1 seconds of arc at 318 Mlix. This limiting size is proportional to the square root of the distance and inversely proportional to the square of the observing frequency. The view that the compact source in the Crab Nebula is the pulsar NP 0532 which has been broadened by interstellar scattering is consistent with these measurements.