# Reverse Shocks in the Relativistic Outflows of Gravitational Wave Detected Neutron Star Binary Mergers

## TL;DR

This paper investigates the role of reverse shocks in the afterglows of neutron star mergers detected via gravitational waves, highlighting their potential observability and implications for magnetic field strength.

## Contribution

It introduces the significance of reverse shocks in off-axis and structured outflows, which were previously neglected, and discusses their observational signatures in early radio afterglows.

## Key findings

- Reverse shocks can cause brightening in early radio afterglows.
- Observable reverse shock emission requires specific inclination angles.
- Strong magnetic fields enhance reverse shock emission, revealing central engine properties.

## Abstract

The afterglows to gamma-ray bursts (GRBs) are due to synchrotron emission from shocks generated as an ultra-relativistic outflow decelerates. A forward and a reverse shock will form, however, where emission from the forward shock is well studied as a potential counterpart to gravitational wave-detected neutron star mergers the reverse shock has been neglected. Here, we show how the reverse shock contributes to the afterglow from an off-axis and structured outflow. The off-axis reverse shock will appear as a brightening feature in the rising afterglow at radio frequencies. For bursts at $\sim100$ Mpc, the system should be inclined $\lesssim20^\circ$ for the reverse shock to be observable at $\sim0.1-10$ days post-merger. For structured outflows, enhancement of the reverse shock emission by a strong magnetic field within the outflow is required for the emission to dominate the afterglow at early times. Early radio photometry of the afterglow could reveal the presence of a strong magnetic field associated with the central engine.