Abstract
The millisecond pulsar J1713+0747 underwent a sudden and significant pulse
shape change between April 16 and 17, 2021 (MJDs 59320 and 59321).
Subsequently, the pulse shape gradually recovered over the course of several
months. We report the results of continued multi-frequency radio observations
of the pulsar made using the Canadian Hydrogen Intensity Mapping Experiment
(CHIME) and the 100-meter Green Bank Telescope (GBT) in a two-year period
encompassing the shape change event, between February 2020 and March 2022. The
amplitude of the shape change and the accompanying TOA residuals display a
strong non-monotonic dependence on radio frequency, demonstrating that the
event is neither a glitch (the effects of which should be independent of
frequency) nor a simple change in electron density along the line of sight (the
effects of which should depend monotonically on frequency). However, it does
bear some resemblance to the two previous "chromatic timing events" observed in
J1713+0747 (Demorest et al. 2013; Lam et al. 2016), as well as to a similar
event observed in PSR J1643-1224 in 2015 (Shannon et al. 2016).