Abstract
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,
multi-epoch scattering times for the large dispersion measure millisecond
pulsar 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, $\tau$, at a fiducial frequency of 1500 MHz changes by
approximately 10% over a 5.5yr 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 $\tau$ vs. 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 $\tau$, manifesting as changes in
the PBF shape and $1/e$ time scale. Finally, we discuss the implications of our
scattering results on pulsar timing including time of arrival delays and
dispersion measure misestimation.