Evidence for mild deviation from power-law distribution of electrons in relativistic shocks: GRB 090902B

TL;DR

This study analyzes the late afterglow of GRB 090902B, revealing that a slight curvature in the electron energy distribution, rather than a pure power-law, better explains the observed X-ray and optical data within the external forward shock model.

Contribution

It demonstrates that a deviation from a simple power-law electron distribution is necessary to explain GRB afterglow observations, linking particle acceleration models with observational data.

Findings

Electron distribution shows slight curvature downstream of shock.

Magnetic energy fraction is about 10^{-6}, consistent across data.

Early high-energy emission aligns with external shock origin.

Abstract

Many previous studies have determined that the long lasting emission at X-ray, optical and radio wavelengths from gamma-ray bursts (GRBs), called the afterglow, is likely produced by the external forward shock model. In this model, the GRB jet interacts with the circum-stellar medium and drives a shock that heats the medium, which radiates via synchrotron emission. In this work, we carried out a detailed analysis of the late time afterglow data of GRB 090902B using a very careful accounting of the Inverse Compton losses. We find that in the context of the external forward shock model, the only viable option to explain the X-ray and optical data of GRB 090920B is to have the electron energy distribution deviate from a power-law shape and exhibit some slight curvature immediately downstream of the shock front (we explored other models that rely on a single power-law assumption, but they all fail to explain the observations). We find the fraction of the energy of shocked plasma in magnetic field to be ~10^{-6} using late time afterglow data, which is consistent with the value obtained using early gamma-ray data. Studies like the present one might be able to provide a link between GRB afterglow modeling and numerical simulations of particle acceleration in collisionless shocks. We also provide detailed calculations for the early (< 10^3 s) high energy (> 100 MeV) emission and confirm that it is consistent with origin in the external forward shock. We investigated the possibility that the ~10 keV excess observed in the spectrum during the prompt phase also has its origin in the external shock and found the answer to be negative.