Closure Relations of Synchrotron Self-Compton in Afterglow stratified medium and Fermi-LAT Detected Gamma-Ray Bursts

TL;DR

This paper derives closure relations for synchrotron self-Compton afterglow models in intermediate density media to better explain high-energy gamma-ray bursts detected by Fermi-LAT, extending previous models to more complex environments.

Contribution

It introduces closure relations for SSC afterglow models in intermediate density profiles, enhancing the understanding of GRB afterglows beyond simple wind or homogeneous media.

Findings

The model explains a subset of GRBs not fitting previous models.

Best-fit stratification parameter is k=0.5 without energy injection.

K=2.5 fits scenarios with energy injection.

Abstract

The Second Gamma-ray Burst Catalog (2FLGC) was announced by the Fermi Large Area Telescope (Fermi-LAT) Collaboration. It includes 29 bursts with photon energy higher than 10 GeV. Gamma-ray burst (GRB) afterglow observations have been adequately explained by the classic synchrotron forward-shock model, however, photon energies greater than 10 GeV from these transient events are challenging, if not impossible, to characterize using this afterglow model. Recently, the closure relations (CRs) of the synchrotron self-Compton (SSC) forward-shock model evolving in a stellar wind and homogeneous medium was presented to analyze the evolution of the spectral and temporal indexes of those bursts reported in 2FLGC. In this work, we provide the CRs of the same afterglow model, but evolving in an intermediate density profile ($\propto {\rm r^{-k}}$) with ${\rm 0\leq k \leq2.5}$, taking into account the adiabatic/radiative regime and with/without energy injection for any value of the electron spectral index. The results show that the current model accounts for a considerable subset of GRBs that cannot be interpreted in either stellar-wind or homogeneous afterglow SSC model. The analysis indicates that the best-stratified scenario is most consistent with ${\rm k=0.5}$ for no-energy injection and ${\rm k=2.5}$ for energy injection.