Title: The Frequency-Dependent Scattering of Pulsar J1903+0327
Abstract: Free electrons in the interstellar medium refract and diffract radio waves along multiple paths, resulting in distortions of radio pulses. Pulsar J1903+0327, the pulsar observed by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav) with the highest dispersion measure (integrated line-of-sight electron density), is one of several pulsars now shown to exhibit peculiar signatures of scattering: not only does the characteristic scattering timescale change with time but the frequency dependence of scattering changes with time as well. We find that the scaling law for the scattering timescale versus radio frequency is strongly affected by, for example, any mismatch between the true and assumed pulse broadening function, which quantifies the changes to the intrinsic pulse shape as it propagates through the interstellar medium. We show using simulations that refraction manifests as changes in the shape of the pulse broadening function and is a plausible cause of the epoch dependence of the scattering timescale and spectral indices. I will briefly discuss the implications for precision pulsar timing and studies of the ionized interstellar medium.