An anisotropic minijets model for the GRB prompt emission

TL;DR

This paper proposes an anisotropic minijets model for GRB prompt emission, explaining rapid variability, delayed afterglow peaks, and steep flux declines by considering minijets oriented perpendicular to the main flow, contrasting with isotropic models.

Contribution

It introduces a novel anisotropic minijets framework that accounts for observed GRB features, differing from previous isotropic assumptions.

Findings

Delayed emission peaks consistent with observations.

Steeper flux decline after source turns off.

Explains timing of GeV and X-ray light curves.

Abstract

In order to explain rapid light curve variability without invoking a variable source, several authors have proposed "minijets" that move relativistically relative to the main flow of the jet. Here we consider the possibility that these minijets, instead of being isotropically distributed in the comoving frame of the jet, form primarily perpendicular to the direction of the flow, as the jet dissipates its energy at a large emission radius. This yields two robust features. First, the emission is significantly delayed compared with the isotropic case. This delay allows for the peak of the afterglow emission to appear while the source is still active, in contrast to the simplest isotropic model. Secondly, the flux decline after the source turns off is steeper than the isotropic case. We find that these two features are realized in gamma-ray bursts (GRBs): 1. The peak of most GeV light curves (ascribed to the external shock) appears during the prompt emission phase. 2. Many X-ray light curves exhibit a period of steep decay, which is faster than that predicted by the standard isotropic case. The gamma-ray generation mechanism in GRBs, and possibly in other relativistic flows, may therefore be anisotropic.