Multiband Fitting to Three Long GRBs with Fermi/LAT Data: Structured Ejecta Sweeping up a Density-Jump Medium

TL;DR

This study models the broadband afterglow emissions of three long GRBs detected by Fermi, proposing that structured ejecta in a density-jump medium explains their high-energy and late-time emissions, with implications for the circumburst environment.

Contribution

It introduces a comprehensive broadband fitting model that accounts for structured ejecta and density jumps, explaining diverse afterglow features of long GRBs detected by Fermi.

Findings

Structured ejecta with continuous energy injection explains early GeV rise.

Density jumps account for late afterglow plateaus and flares.

Low-density environments suppress SSC radiation, affecting high-energy emission.

Abstract

We present broadband (radio, optical, X-ray and GeV) fits to the afterglow light curves and spectra of three long-duration gamma-ray bursts (GRBs 080916C, 090902B, and 090926A) detected by the Gamma-Ray Burst Monitor (GBM) and Large Area Telescope (LAT) instruments on the Fermi satellite. Using the observed broadband data, we study the origin of the high energy emission, and suggest that the early-time GeV emission and the late-time radio, optical, and X-ray afterglows can be understood as being due to synchrotron emission from an external forward shock caused by structured ejecta propagating in a wind bubble jumping to a homogeneous density medium. If the ceasing time for majority of the energy injection is assumed to be close to the deceleration time of the forward shock, the structured ejecta with continuous energy injection to the forward shock can well explain the early rising feature of the GeV mission from these burst, and the density-jump medium can account for some certain plateaus or flares in the late afterglows. From our fits, we find that, on one hand, the external shock origin of the GeV photons will make the optical depth have not significant contribution to the early LAT rising part, which will loosen strong constraint of lower limits of Lorentz factor. On the other hand, these Fermi-LAT events preferentially occur in a low-density circumburst environment, in which case the Klein-Nishina cutoff will significantly suppress the Self-Synchrotron Compton (SSC) radiation. Such an environment might result from superbubbles or low-metallicity progenitor stars (which have a low mass-loss rate at late times of stellar evolution) of type Ib/c supernovae.