The NANOGrav 12.5 yr Data Set: Probing Interstellar Turbulence and Precision Pulsar Timing with PSR J1903+0327

Free electrons in the interstellar medium refract and diffract radio waves along multiple paths, resulting in angular and temporal broadening of radio pulses that limits pulsar timing precision. We determine multifrequency, multiepoch scattering times for the large dispersion measure millisecond PSR J1903+0327 by developing a three-component model for the emitted pulse shape that is convolved with a best-fit pulse broadening function (PBF) identified from a family of thin-screen and extended-media PBFs. We show that the scattering time, τ, at a fiducial frequency of 1500 MHz changes by approximately 10% over a 5.5 yr span with a characteristic timescale of approximately 100 days. We also constrain the spectral index and inner scale of the wavenumber spectrum of electron density variations along this line of sight. We find that the scaling law for τ versus radio frequency is strongly affected by any mismatch between the true and assumed PBF or between the true and assumed intrinsic pulse shape. We show using simulations that refraction is a plausible cause of the epoch dependence of τ, manifesting as changes in the PBF shape and 1/e timescale. Finally, we discuss the implications of our scattering results on pulsar timing including time of arrival delays and dispersion measure misestimation.