Polarization Properties of Millisecond Pulsars: Astrophysical Interpretations and Applications

Pulsars are some of the most extreme objects in the universe; their small yet incredibly predictable spin periods coupled with their strong magnetic fields make them ideal laboratories for study. Not only are they interesting objects themselves, but they can also help us probe different astrophysical environments, such as the interstellar magnetic field and the solar corona. These stars are highly polarized, and that polarization comes into play in various fields of pulsar physics (such as constraining models of pulsar emission), but obtaining that polarization information can be difficult, as the polarization properties of the light can change as the radio waves move through our telescope receivers. In this work, we perform polarimetric calibrations for 23 millisecond pulsars at 820 and 1500 MHz with the Green Bank Telescope as part of the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) pulsar timing array. We calibrate the data using Mueller matrix solutions calculated from observations of PSRs B1929+10 and J1022+1001. We present their polarization profiles, some of which have not been previously published. Through study of these very high signal-to-noise profiles, we also discovered very low intensity average profile components (“microcomponents”) in four pulsars. We obtain the Faraday rotation measures for each pulsar and use them to calculate the Galactic magnetic field parallel to the line of sight for different lines of sight through the interstellar medium. We fit for linear and sinusoidal trends in time in the dispersion measure and Galactic magnetic field and detect magnetic field variations with a period of one year in some pulsars, but overall find that the variations in these parameters are more consistent with a stochastic origin. In addition to being used to study the interstellar medium, pulsars can also be used to search for low-frequency gravitational waves. Because these perturbations are small, very precise models are needed to account for various sources of noise in the data set. One of those sources of noise could come from polarization calibration, as only a very basic calibration is done before the data is compared to the pulsar models. We took three pulsars with different polarization fractions from our previous study and put their polarization profiles through the NANOGrav timing pipeline to see the effect the robust polarization calibration has on the data. We found that the RMS (the root mean square of the residuals, the difference between the model and the data) did improve in some cases, and that the timing parameters were affected for each pulsar. The most significant improvement was for PSR J1643−1224, which has a large rotation measure. Our study represents a very small portion of the data available but the possibility of improvements. In order to quantify how much robust polarization calibration increases our sensitivity to gravitational waves, a full analysis using NANOGrav’s more recent timing pipeline is needed