Presentation #401.05 in the session “What Do We Learn from Prompt Observations of Transient Astrophysical Events?”.
The first light that emerges from a supernova is the shock breakout emission, which in the spherical case is characterized by a brief UV flash. In the case of an axisymmetric, prolate explosion, the shock first breaches the surface along the symmetry axis, then marches towards the equator; the resulting breakout light curve may differ substantially from the spherically symmetric case. We study the properties of the bolometric light curve associated with the emergence of a non-relativistic, bipolar shock from a spherical star, and identify four possible classes of breakout light curves, depending on the degree of asphericity. Compared to spherical breakouts, we find that significantly aspherical breakouts are distinguished by 1) a longer and fainter initial breakout flash and 2) an extended phase of slowly-declining emission which is produced as ejecta flung sideways during the breakout expand and cool. We show that, even when allowing for asphericity, the duration of the breakout flash cannot exceed roughly ~R∗/vbo, where R∗ is the stellar radius and vbo is the velocity of the fastest-moving ejecta. Applying this result, we find that the long duration of the X-ray flash seen in SN 2008D cannot be explained as an aspherical breakout from a standard Wolf-Rayet star, and the same is true for the prompt X-ray emission associated with low-luminosity GRBs such as GRB 060218. We therefore suggest that these events originate from non-standard progenitors with larger radii.