Evidence for Cocoon Emission from the Early Light Curve of SSS17a

TL;DR

This paper presents evidence that early blue emission in the neutron star merger event SSS17a is caused by cocoon emission resulting from shock cooling of jet-interacted debris, aiding future detection strategies.

Contribution

It introduces the concept that early blue light from neutron star mergers can be explained by cocoon emission from shock cooling, with formulas for future observational studies.

Findings

Early blue emission explained by shock cooling of cocoon.

Formulas provided for studying shock cooling in future events.

Highlights importance of early optical surveys for neutron star mergers.

Abstract

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. We summarize the properties of this emission and provide formulae that can be used to study future detections of shock cooling from merging neutron stars. This argues that optical transient surveys should search for such early, blue light if they wish to find off-axis gamma-ray bursts and double neutron star gravitational wave events as soon as possible after the merger.