Moving-mesh simulations of spreading dynamics and local electron cooling in structured gamma-ray burst afterglow jets

TL;DR

This paper uses moving-mesh simulations to study structured gamma-ray burst jets, revealing how jet spreading and electron cooling influence observed spectra and jet break features.

Contribution

It introduces a local cooling approach in simulations, showing its significant effects on the synchrotron cooling break and spectral smoothness compared to global cooling methods.

Findings

Jet spreading is influenced by initial angular structure but diminishes over time.

Local cooling causes a substantial upward shift and smoothing of the cooling break in spectra.

Jet breaks become sharper at higher frequencies, with local cooling leading to steeper post-break slopes.

Abstract

We present the results for the dynamics and emission profiles of axi-symmetric numerical simulations of structured gamma-ray burst afterglow jets, computed using the relativistic moving-mesh hydrodynamics code GAMMA. We find that the spreading of jets of average opening angle is moderately impacted by the initial steepness of the angular structure, although the effect disappears once the working surface of the jet substantially exceeds its initial width, and that the travel time of a sound wave across the front surface remains the best indicator of the onset of spreading also for structured jets. When computing the afterglow spectrum using a local cooling approach that traces the electron population following shock-acceleration, we observe a significant impact on the synchrotron cooling break. Similar to earlier results for top-hat jets, the cooling break is found to shift upward in frequency by well over a factor of ten relative to approaches that assume a global cooling timescale across the jet. The cooling break transition in the spectrum also becomes substantially smoother. For both local and global cooling, jet breaks become sharper with increasing frequency. Local cooling is found to initially lead to a steeper slope post jet-break. The local-cooling emission is shown to originate from a narrow frequency-dependent sized region behind the shock front, as expected, but in strong contrast to a global cooling approach.