Time Variability of Interstellar Scattering and Improvements to Pulsar Timing

Abstract

Delay due to multipath scattering in the interstellar medium is a concern for high-precision pulsar timing, particularly if it is not constant over time. We report on 36 weekly observations of the pulsar PSR B1737+13 with the Arecibo telescope that monitored the time variability of the scattering delay. At a frequency of 1380 MHz, the interstellar delay varied between 0.2 and 2.1 μs (+/-0.1 μs) over 270 days of observation. The delay was consistent over four observing bands with center frequencies from 1175 to 1470 MHz and scaled as τ~ν-3.6+/-0.2, which differs from the ν-4.4 scaling expected for Kolmogorov turbulence. We show that another estimation technique is feasible for weaker pulsars or smaller telescopes, although it underestimates the delay during episodes of extra scattering detectable through a full secondary spectrum analysis. An array of pulsars distributed around the sky can be used as a sensitive detector of long-wavelength (~ several light-years) gravitational radiation, and such pulsar timing array observations have been initiated by several groups worldwide. To reach interesting sensitivity levels it is necessary to reduce the sources of error to below 1 μs, and 100 ns is a target precision level. Correction for interstellar scattering delay will be an important step in achieving long-term, submicrosecond timing precision.