The frequency dependence of radio pulse arrival times provides a probe of
structures in the intervening media. Demorest et al. 2013 was the first to show
a short-term (~100-200 days) reduction in the electron content along the line
of sight to PSR J1713+0747 in data from 2008 (approximately MJD 54750) based on
an apparent dip in the dispersion measure of the pulsar...
We report on a similar
event in 2016 (approximately MJD 57510), with average residual pulse-arrival
times of approximately 3.0,-1.3, and -0.7 microseconds at 820, 1400, and 2300
MHz, respectively. Timing analyses indicate possible departures from the
standard nu^-2 dispersive-delay dependence. We discuss and rule out a wide
variety of potential interpretations. We find the likeliest scenario to be
lensing of the radio emission by some structure in the interstellar medium,
which causes multiple frequency-dependent pulse arrival-time delays.