Precision measurements of pulsar dispersion

Abstract

Timing observations of eight pulsars over a frequency range from 25 MHz to 5 GHz are performed in order to investigate possible departures from the nu exp -2 dispersion law that applies to the 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. The absence of LF deviations from a nu exp -2 dispersion law at the 1-ms level is consistent with a Kolmogorov spectrum of the interstellar plasma turbulence extending to scale sizes of about 10 exp 15 cm. Forms of the interstellar dispersion law which included nu exp -3 and nu exp -4 terms arising from clumping, magnetic fields, and temperature effects in the dispersing gas were examined. Pulsar dispersion was found to be an insensitive probe of gas temperature, even for a hot plasma. Dispersion delays varying as nu exp -4 could be detected at decameter wavelengths if the line of sight passes through a dense H II region.