# The cocoon emission - an electromagnetic counterpart to gravitational waves from neutron star mergers

## TL;DR

This paper models the cocoon emission from neutron star mergers, predicting a bright, short-lived blue optical signal as an electromagnetic counterpart to gravitational waves, aiding future detection efforts.

## Contribution

The study provides 3D simulations of jet propagation and emission in neutron star mergers, highlighting the potential observability of cocoon signals as counterparts to gravitational wave events.

## Key findings

- Cocoon emission produces a bright (~ -14 to -15 mag) short (~1 hour) blue optical signal.
- The brightness of the cocoon signal is influenced by jet breakout time, less so by opacity.
- The predicted signal outshines other UV-optical counterparts, making it a promising detection target.

## Abstract

Short Gamma-Ray Bursts (SGRBs) are believed to arise from compact binary mergers (either neutron star-neutron star or black hole-neutron star). If so their jets must penetrate outflows that are ejected during the merger. As a jet crosses the ejecta it dissipates its energy, producing a hot cocoon which surrounds it. We present here 3D numerical simulations of jet propagation in mergers' outflows and we calculate the resulting emission. This emission consists of two components: the cooling emission, the leakage of the thermal energy of the hot cocoon, and the cocoon macronova that arises from the radioactive decay of the cocoon's material. This emission gives a brief (~ one hour) blue, wide angle signal. While the parameters of the outflow and jet are uncertain, for the configurations we have considered the signal is bright (~ -14 $-$ -15 absolute magnitude) and outshines all other predicted UV-optical signals. The signal is brighter when the jet breakout time is longer and its peak brightness does not depend strongly on the highly uncertain opacity. A rapid search for such a signal is a promising strategy to detect an electromagnetic merger counterpart. A detected candidate could be then followed by deep IR searches for the longer but weaker macronova arising from the rest of the ejecta.