| Abstract: |
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. However, if the explosion is asymmetric, the observed breakout emission may differ substantially from the spherically symmetric case. I will give an overview of our recent work on generalizing the standard shock breakout theory to the case of axisymmetric explosions, mainly focusing on how asymmetry affects the bolometric light curve. 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_∗/v_{bo}, where R_∗ is the stellar radius and v_{bo} 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. I will also briefly mention recent progress on the spectrum and temperature evolution of aspherical shock breakouts, and discuss how early multi-wavelength observations might be able to probe explosion asymmetry in the future. |
