Evidence for Cocoon Emission from the Early Light Curve of SSS17a

Understand

Swope Supernova Survey 2017a (SSS17a) was discovered as the first optical counterpart to the gravitational wave event GW170817.

• Although its light curve on the timescale of weeks roughly matches the expected luminosity and red color of an r-process powered transient, the explanation for the blue emission from high velocity material over the first few days is not as clear.
• Here we show that the power-law evolution of the luminosity, temperature, and photospheric radius during these early times can be explained by cooling of shock heated material around the neutron star merger.
• This heating is likely from the interaction of the gamma-ray burst jet with merger debris, so-called cocoon emission.