A Global Numerical Model of the Prompt Emission in Short Gamma-ray Bursts

We present the first global model of prompt emission from a short gamma-ray burst (GRB) that consistently describes the evolution of the central black hole (BH) torus system, the propagation of the jet through multicomponent merger ejecta, the transition into free expansion, and the photospheric emission from the relativistic jet. To this end, we perform a special relativistic neutrino-hydrodynamics simulation of a viscous BH-torus system, which is formed about 500 ms after the merger and is surrounded by dynamical ejecta as well as neutron star winds, along with a jet that is injected in the vicinity of the central BH. In a postprocessing step, we compute the photospheric emission using a relativistic Monte Carlo radiative transfer code. It is found that the wind from the torus leaves a strong imprint on the jet as well as on the emission, causing narrow collimation and rapid time variability. The dependence of the emission on viewing angle gives rise to correlations among the spectral peak energy, E ( p ), isotropic energy, E (iso), and peak luminosity, L ( p ), which may provide natural explanations for the Amati and Yonetoku relations. We also find that the degree of polarization is small for emission from the jet core (less than or similar to 2%), while it tends to increase with viewing angle outside the core and can become as high as similar to 10%-40% for energies larger than the peak energy. Finally, the comparison of our model with GRB 170817A strongly disfavors the photospheric emission scenario and therefore supports alternative scenarios, such as cocoon shock breakout.

Automated summary

This study presents a global model of prompt emission from a short gamma-ray burst, describing the evolution of the central black hole torus system, jet propagation, transition into free expansion, and photospheric emission. The modeling shows the impact of the torus wind on the jet and emission, leading to correlations among spectral peak energy, isotropic energy, and peak luminosity. The comparison with GRB 170817A disfavors the photospheric emission scenario and supports alternative scenarios like cocoon shock breakout.