  The shock breakout emission is the first light that emerges from a supernova.
In the spherical case it is characterized by a brief UV flash. In an
axisymmetric, non-spherical prolate explosion, the shock first breaches the
surface along the symmetry axis, then peels around to larger angles, producing
a breakout light curve which may differ substantially from the spherically
symmetric case. We study the emergence of a non-relativistic, bipolar shock
from a spherical star, and estimate the basic properties of the associated
bolometric shock breakout signal. We identify four possible classes of breakout
light curves, depending on the degree of asphericity. Compared to spherical
breakouts, we find that the main distinguishing features of significantly
aspherical breakouts are 1) a longer and fainter initial breakout flash and 2)
an extended phase of slowly-declining, or even rising, emission which is
produced as ejecta flung out by the oblique breakout expand and cool. We find
that the breakout flash has a maximum duration of roughly $\sim R_*/v_{\rm bo}$
where $R_*$ is the stellar radius and $v_{\rm bo}$ is the velocity of the
fastest-moving ejecta. For a standard Wolf--Rayet progenitor, the duration of
the X-ray flash seen in SN 2008D exceeds this limit, and the same holds true
for the prompt X-ray emission of low-luminosity GRBs such as GRB 060218. This
suggests that these events cannot be explained by an aspherical explosion
within a typical Wolf--Rayet star, implying that they originate from
non-standard progenitors with larger breakout radii.
