A first-principle simulation of blast wave emergence at the photosphere of a neutron star merger

TL;DR

This paper presents the first ab initio simulation of a radiation-mediated shock at a neutron star merger's photosphere, reproducing the gamma-ray burst observed in GW170817 with detailed radiative transfer modeling.

Contribution

It introduces a novel simulation code that couples fluid dynamics with radiative transfer to model shock emergence in neutron star mergers, providing new insights into gamma-ray burst production.

Findings

The simulation reproduces the observed gamma-ray burst light curve.

It shows how shocks generate and release radiation at the photosphere.

The spectral evolution matches that of GRB 170817A.

Abstract

We present the first ab initio simulation of a radiation-mediated shock emerging at the photosphere of a relativistic outflow. The simulation is performed using our code radshock that follows fluid dynamics coupled to time-dependent radiative transfer, calculated with the Monte-Carlo method. We use the code to examine the radiative blast wave emerging from neutron star merger GW~170817. It was previously proposed that the merger ejected a dark, relativistically expanding, homologous envelope, and then an explosion inside the envelope produced the observed gamma-ray burst GRB~170817A. Our simulation demonstrates how the shock wave generates radiation as it propagates through the envelope, approaches its photosphere, releases the radiation, and collapses, splitting into two collisionless shocks of a microscopic thickness. We find the light curve and the spectral evolution of the produced gamma-ray burst; both are similar to the observed GRB~170817A.