PRECISION-MEASUREMENTS OF PULSAR DISPERSION

We have conducted timing observations of eight pulsars over a frequency range from 25 MHz to 5 GHz to investigate possible departures from the nu-2 dispersion law that applies to propagation of radio waves through the tenuous interstellar plasma. Apparent deviations from a cold plasma law were found at high frequencies (2-5 GHz) for two pulsars: PSR 0525 + 21 and PSR 1237 + 25. No such departures were detected in pulse arrival times at meter or decameter wavelengths, within measurement uncertainties of approximately 1 ms. The absence of low-frequency deviations from a nu-2 dispersion law at the 1 ms level was consistent with a Kolmogorov spectrum (power law with 11/3 index) of the interstellar plasma turbulence extending to scale sizes approximately 10(15) cm. Average-profile techniques at 25 and 47 MHz were not sensitive enough to measure timing delays associated with interstellar refraction. We showed that they are potentially detectable by timing microfeatures in single pulses below 50 MHz. We examined forms of the interstellar dispersion law which included nu-3 and nu-4 terms arising from clumping, magnetic fields, and temperature effects in the dispersing gas. Pulsar dispersion was found to be an insensitive probe of gas temperature, even for a hot plasma. Dispersion delays is-proportional-to nu-4 could be detected at decameter wavelengths if the line of sight passes through a dense H II region.