Temporal variations in scattering and dispersion measure in the Crab Pulsar and their effect on timing precision

TL;DR

This study analyzes 30 years of Crab Pulsar observations to understand how scattering and dispersion measure variations, caused by structures within the nebula, impact timing precision and how correcting for these effects can improve pulsar timing accuracy.

Contribution

It provides the first long-term analysis linking scattering and dispersion measure variations in the Crab Pulsar and demonstrates a method to improve timing precision by correcting for scattering effects.

Findings

Correlation coefficient between scattering and dispersion measure variations is 0.56.

Discrete nebular structures of about 6 AU cause observed variations.

Applying scattering corrections doubles the timing residual accuracy.

Abstract

We have measured variations in scattering time scales in the Crab Pulsar over a 30-year period, using observations made at 610 MHz with the 42-ft telescope at Jodrell Bank Observatory. Over more recent years, where regular Lovell Telescope observations at frequencies around 1400 MHz were available, we have also determined the dispersion measure variations, after disentangling the scattering delay from the dispersive delay. We demonstrate a relationship between scattering and dispersion measure variations, with a correlation coefficient of $0.56\pm0.01$. The short time scales over which these quantities vary, the size of the variations, and the close correlation between scattering and dispersion measure all suggest that the effects are due to discrete structures within the Crab Nebula, with size scales of $\sim6$ AU (corresponding to an angular size of $\sim2$ mas at an assumed distance of 2200 pc). We mitigate the effects of scattering on the observed pulse shape by using the measured scattering information to modify the template used for generating the pulse arrival times, thus improving the precision to which the pulsar can be timed. We test this on timing data taken during periods of high scattering, and obtain a factor of two improvement in the root mean square of the timing residuals.