Characterizing & Mitigating Interstellar Scattering in Radio Observations of Pulsars

Pulsars are rapidly rotating neutron stars that emit concentrated beams of radiation from their magnetic poles. Measuring the arrival of these pulses via pulsar timing is a powerful tool in efforts to detect low frequency gravitational waves (GWs). However, the interstellar medium (ISM) presents a significant source of timing uncertainty that must be mitigated to improve the sensitivity of pulsar timing efforts to these GW signals. By examining the effects of pulsar emission interactions with this medium, we can properly correct for the resulting effects in pulsar timing efforts, as well as study the astronomical unit-to-parsec scale structure and behavior of the ISM and characterize this medium across many lines of sight in our Galaxy and localize the regions that dominate these interactions. Emerging data processing techniques that take advantage of the periodic nature of pulsar signals are also now allowing us to probe these features with incredible resolution. This thesis serves to highlight some valuable studies related to understanding the structure and behavior of the ionized ISM via the interstellar scattering of pulsar emission. To examine scattering behavior across many lines of sight, we extract interstellar scintillation parameters for pulsars observed by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) radio pulsar timing program in the 12.5 year data release. We find good agreement between our scattering delay measurements and electron- density model predictions for most pulsars. For most pulsars for which scattering delays are 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 currently subdominant in our overall noise budget but are important for achieving precisions of tens of nanoseconds or less. Next, we use the Upgraded Giant Metrewave Radio Telescope to measure scin- tillation arc properties in six bright canonical pulsars with simultaneous dual frequency coverage. We perform more robust determinations of arc curvature, scattering delay, and scintillation timescale frequency-dependence, and comparison between arc curvature and pseudo-curvature than allowed by single-frequency-band-per-epoch measurements, which we find to agree with theory and previous literature. We find a strong correlation between arc asymmetry and arc curvature, which we have replicated using simulations, and attribute to a bias in the Hough transform approach to scintillation arc analysis. We then simulate scattering delays from the ISM to examine the effectiveness of three estimators in recovering these delays in pulsar timing data. Two of these estima- tors use the more traditional process of fitting autocorrelation functions (ACFs) to pulsar dynamic spectra to extract scintillation bandwidths, while the third estimator uses the newer technique of cyclic spectroscopy on baseband pulsar data to recover the ISM’s im- pulse response function (IRF). We find that, given sufficient S/N, cyclic spectroscopy is more accurate than both ACF estimators at recovering scattering delays at specific epochs, suggesting that cyclic spectroscopy is a superior method for scattering estimation in high quality data. Finally, we use cyclic spectroscopy to perform high frequency-resolution, frequency dependent analyses of the millisecond pulsar B1937+21. We present among the most robust intra-epoch scattering delay scaling estimations performed at 1.4 GHz, using eight individual measurements across our observing bands, and find our results to agree with those previously quoted in the literature