  We examine flash spectroscopy of a circumstellar medium (CSM) ionized by the
hard radiation pulse produced by the emerging shock of a supernova (SN). We
first find that the rise and fall times of the Halpha emission constrains the
location of the CSM with a peak at tpeak=Rstar sqrt(2/c vshock) for a star of
radius Rstar and a shock velocity of vshock. The dropping temperature of the
transient emission naturally reproduces the evolution of lines with different
ionization energies. Second, for red supergiants (RSGs), the shock break out
radiatively accelerates the CSM to produce broad, early-time line wings
independent of the Thomson optical depth of the CSM. Finally, the CSM
recombination rates in binaries can be dominated by a dense, cool, wind
collision interface like those seen in Wolf-Rayet binaries rather than the
individual stellar winds. Combining these three results, the flash spectroscopy
observations of the normal Type IIP iPTF13dqy (SN 2013fs) are naturally
explained by an RSG with a normal, Thomson optically thin wind in a binary with
a separation of ~10^4 Rsun without any need for a pre-SN eruption. Similarly,
the broad line wings seen for the Type IIb iPTF13ast (SN 2013cu), whose
progenitors are generally yellow supergiants in binaries, are likely due to
radiative acceleration of the CSM rather than pre-existing, Wolf-Rayet-like
wind.
