Interstellar Scintillation and the Radio Counterpart of the Fast Radio
  Burst FRB150418
  Keane et al. (2016) have recently reported the discovery of a new fast radio
burst, FRB150418, with a promising radio counterpart at 5.5 and 7.5 GHz -- a
rapidly decaying source, falling from 200-300 $\mu$Jy to 100 $\mu$Jy on
timescales of $\sim$6 d. This transient source may be associated with an
elliptical galaxy at redshift $z=0.492$, providing the first firm spectroscopic
redshift for a FRB and the ability to estimate the density of baryons in the
intergalactic medium via the combination of known redshift and radio dispersion
of the FRB. An alternative explanation, first suggested by Williams & Berger
(2016b), is that the identified counterpart may instead be a compact AGN. The
putative counterpart's variation may then instead be extrinsic, caused by
refractive scintillation in the ionized interstellar medium of the Milky Way,
which would invalidate the association with FRB150418. We examine this latter
explanation in detail and show that the reported observations are consistent
with scintillating radio emission from the core of a radio-loud active galactic
nucleus (AGN) having a brightness temperature $T_{\rm b} \gtrsim 10^9 {\rm K}$.
Using numerical simulations of the expected scattering for the line of sight to
FRB150418, we provide example images and light curves of such an AGN at 5.5 and
7.5 GHz. These results can be compared with continued radio monitoring to
conclusively determine the importance of scintillation for the observed radio
variability, and they show that scintillation is a critical consideration for
continued searches for FRB counterparts at radio wavelengths.
