Variation of Microphysical Parameters in Reverse-shock Scenario

TL;DR

This paper investigates how varying microphysical parameters in the reverse shock of gamma-ray bursts affect observed light curves and spectral evolution, providing a more flexible model that explains diverse observational features.

Contribution

It introduces a model allowing microphysical parameters to vary over time in reverse shocks, improving understanding of GRB light curves and spectral behaviors.

Findings

Varying microphysical parameters can produce plateau phases in GRB light curves.

The model explains steeper decay indices than high-latitude emission predictions.

Application to Fermi-LAT GRB data supports the model's relevance.

Abstract

Gamma-ray bursts (GRBs), among the most compelling astrophysical phenomena, are potential candidates for exploring the evolution of energy distribution among magnetic fields and particles through multiwavelength observations. The fraction of energy transferred between particles and the magnetic field is governed by microphysical parameters, typically assumed to be constant during relativistic shocks but may in fact vary with time. In this work, we derive the light curves and closure relations (CRs) of the synchrotron-self Compton (SSC) process from the external reverse shock (RS) with variations of microphysical parameters in a homogeneous and stellar-wind medium. We consider the evolution of the RS in the thick- and thin-shell regimes. We demonstrate that, depending on the microphysical parameters, this process can mimic plateau phases and produce temporal decay indices steeper than those predicted by high-latitude emission alone. The current model is employed to examine the evolution of the spectral and temporal indices of GRBs reported in the Second Fermi-LAT Gamma-ray Burst Catalog (2FLGC) and bursts detected at very high energies, using Markov Chain Monte Carlo (MCMC) simulations.