The NANOGrav 12.5-Year Data Set: Monitoring Interstellar Scattering Delays

We extract interstellar scintillation parameters for pulsars observed by the NANOGrav radio pulsar timingprogram. Dynamic spectra for the observing epochs of each pulsar were used to obtain estimates of scintillationtimescales, scintillation bandwidths, and the corresponding scattering delays using a stretching algorithm toaccount for frequency-dependent scaling. We were able to measure scintillation bandwidths for 28 pulsars at1500 MHz and 15 pulsars at 820 MHz. We examine scaling behavior for 17 pulsars and find indices rangingfrom -0:7 to -3:6. We were also able to measure scintillation timescales for six pulsars at 1500 MHz and sevenpulsars at 820 MHz. There is fair agreement between our scattering delay measurements and electron-densitymodel predictions for most pulsars, with some significant outliers likely resulting from frequency channel resolutionlimits outside of this range. We derive interstellar scattering-based transverse velocities assuming ascattering screen halfway between the pulsar and earth. We also calculate the location of the scattering screensassuming proper motion and interstellar scattering-derived transverse velocities are equal. We find no correlationsbetween variations in scattering delay and either variations in dispersion measure or flux density. For mostof the pulsars for which scattering delays were measurable, we find that time of arrival uncertainties for a given epoch are larger than our scattering delay measurements, indicating that variable scattering delays are currentlysubdominant in our overall noise budget but are important for achieving precisions of tens of ns or less.