The peculiar light-curve of J1415+1320: A case study in extreme
  scattering events
  The radio light-curve of J1415+1320 (PKS 1413+135) shows time-symmetric and
recurring U-shaped features across the cm-wave and mm-wave bands. The symmetry
of these features points to lensing by an intervening object as the cause.
U-shaped events in radio light curves in the cm-wave band have previously been
attributed to Extreme scattering events (ESE). ESEs are thought to be the
result of lensing by compact plasma structures in the Galactic interstellar
medium, but the precise nature of these plasma structures remains unknown.
Since the strength of a plasma lens evolves with wavelength $\lambda$ as
$\lambda^2$, the presence of correlated variations at over a wide wavelength
range casts doubt on the canonical ESE interpretation for J1415+1320. In this
paper, we critically examine the evidence for plasma lensing in J1415+1320. We
compute limits on the lensing strength, and the associated free-free opacity of
the putative plasma lenses. We compare the observed and model ESE light curves,
and also derive a lower limit on the lens distance based on the effects of
parallax due to the Earth's orbit around the Sun. We conclude that plasma
lensing is not a viable interpretation for J1415+1320's light curves and that
symmetric U-shaped features in the radio light curves of extragalactic sources
do not present {\em prima facie} evidence for ESEs. The methodology presented
here is generic enough to be applicable to any plasma lensing candidate.
