  Propagation effects in the interstellar medium and intrinsic profile changes
can cause variability in the timing of pulsars, which limits the accuracy of
fundamental science done via pulsar timing. One of the best timing pulsars, PSR
J1713+0747, has gone through two `dip' events in its dispersion measure (DM)
time series. If these events reflect real changes in electron column density,
they should lead to multiple imaging. We show that the events are are well
fitted by an underdense corrugated sheet model, and look for associated
variability in the pulse profile using principal component analysis. We find
that there are transient pulse profile variations, but they vary in concert
with the dispersion measure, unlike what is expected from lensing due to a
corrugated sheet. The change is consistent in shape across profiles from both
the Greenbank and Arecibo radio observatories, and its amplitude appears to be
achromatic across the 820-MHz, 1.4-GHz, and 2.3-GHz bands, again unlike
expected from interference between lensed images. This result is puzzling. We
note that some of the predicted lensing effects would need higher time and
frequency resolution data than used in this analysis. Future events appear
likely, and storing baseband data or keeping multiple time-frequency
resolutions will allow more in-depth study of propagation effects and hence
improvements to pulsar timing accuracy.
