INTERSTELLAR PROPAGATION EFFECTS AND THE PRECISION OF PULSAR TIMING

Interstellar propagation effects induced by electron-density turbulence limit the precision of pulsar timing. We study three distinct arrival time perturbations that are associated with propagation through the ionized component of the interstellar medium. One is due to variations in the electron-column density while angle-of-arrival variations cause the other two. The three effects have different frequency scalings as well as different time dependences for electron-density fluctuations having a wide distribution of length scales. We assess the feasibility of removing propagation effects by analyzing multi-frequency timing measurements. Maximization of the signal-to-noise ratio from weak pulsars forces observations toward low frequencies where the amplitudes of interstellar propagation effects increase. We examine in detail the arrival times from simulated PSR 1937 + 21 data and draw the following conclusions: (1) for a Kolmogorov power-law spectrum, the arrival time errors are dominated by dispersion measure variations at radio frequencies above several hundred Megahertz; (2) dual-frequency observations can be used to remove dispersion measure variations to obtain submicrosecond residuals; (3) the remaining residuals are highly correlated with the angle-of-arrival variations; (4) fitting for a non-λ2 term does improve the final residuals, although it does not appear to improve the measurement of the astrometric terms of proper motion and parallax; and (5) frequent sampling with a small antenna is better for allowing removal of propagation effects and measuring proper motion and parallax in timing data than coarse sampling with a larger antenna (when the number of observations with equal time and bandwidth balances the difference in antenna gain). Simulated parallax measurements are fit within 25% of the "true" value through a turbulent phase screen along only three lines of sight: toward pulsars 1620-26, 1855 + 09, and 1957 + 20.