The NANOGrav Nine-year Data Set: Monitoring Interstellar Scattering Delays

TL;DR

This study monitors interstellar scattering delays in pulsar timing data, revealing variability and scaling behaviors that impact timing precision and inform models of the interstellar medium.

Contribution

It introduces a method to measure and analyze interstellar scattering delays in pulsar timing data, highlighting their variability and implications for timing accuracy.

Findings

Scattering delays vary by up to an order of magnitude over time.

Measured delays are slightly higher than predictions from existing models.

Scattering delays are generally negligible compared to timing uncertainties.

Abstract

We report on an effort to extract and monitor interstellar scintillation parameters in regular timing observations collected for the NANOGrav pulsar timing array. Scattering delays are measured by creating dynamic spectra for each pulsar and observing epoch of wide-band observations centered near 1500 MHz and carried out at the Green Bank Telescope and the Arecibo Observatory. The ~800-MHz wide frequency bands imply dramatic changes in scintillation bandwidth across the bandpass, and a stretching routine has been included to account for this scaling. For most of the 10 pulsars for which the scaling has been measured, the bandwidths scale with frequency less steeply than expected for a Kolmogorov medium. We find estimated scattering delay values that vary with time by up to an order of magnitude. The mean measured scattering delays are similar to previously published values and slightly higher than predicted by interstellar medium models. We investigate the possibility of increasing the timing precision by mitigating timing errors introduced by the scattering delays. For most of the pulsars, the uncertainty in the time of arrival of a single timing point is much larger than the maximum variation of the scattering delay, suggesting that diffractive scintillation remains only a negligible part of their noise budget.