[Paper Review] SSC Emission as Explanation of The Gamma Ray Afterglow Observed in GRB 980923

N. Fraija, Magdalena González+1 more|arXiv (Cornell University)|Oct 28, 2011

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper proposes that the high-energy gamma-ray afterglow in GRB 980923 is explained by inverse Compton scattering (SSC) from electrons in the reverse shock of a highly magnetized, thick-shell fireball. The model successfully reproduces the observed 2-second hard component with a spectral index of −1.44 ± 0.07, peak flux of 21.2 × 10⁻⁶ erg/cm²/s, and break energies around 427.9 MeV, consistent with LAT-era observations and the fast-cooling SSC regime.

ABSTRACT

GRB 980923 was one of the brightest bursts observed by the Burst and Transient Source Experiment (BATSE). Previous studies have detected two distinct components in addition to the main prompt episode, which is well described by a Band function. The first of these is a tail with a duration of 400s, while the second is a high-energy component lasting 2s. After summarizing the observations, we present a model for this event and conclude that the tail can be understood as the early gamma-ray afterglow from forward shock synchrotron emission, while the high-energy component is described by the SSC emission from the reverse shock. The main assumption is that of a thick-shell case from highly magnetized ejecta. The calculated fluxes, break energies, starting times and spectral index are all consistent with the observed values.

Motivation & Objective

• To explain the high-energy gamma-ray afterglow component in GRB 980923, which extends up to 150 MeV and lasts ~2 seconds, beyond the main prompt emission.
• To reconcile the observed spectral index of −1.44 ± 0.07 and flux of 21.2 × 10⁻⁶ erg/cm²/s with a physical emission mechanism in the afterglow phase.
• To test whether synchrotron self-Compton (SSC) emission from the reverse shock in a thick-shell, highly magnetized fireball model can account for the hard component without requiring external injection.
• To unify the interpretation of the smooth 400 s tail (attributed to forward shock synchrotron emission) and the hard 2 s component (attributed to reverse shock SSC) within a single fireball framework.
• To constrain the magnetic field and electron energy equipartition in the reverse shock region using observed spectral and temporal characteristics.

Proposed method

• Assumes a thick-shell fireball model with highly magnetized ejecta, leading to a reverse shock that heats and accelerates electrons.
• Applies the synchrotron self-Compton (SSC) process to upscatter synchrotron photons from the reverse shock to high energies, producing the observed hard component.
• Uses analytical expressions for SSC break frequencies ν(IC)_m and ν(IC)_c, and peak flux F(IC)_max, derived from relativistic electron distributions and magnetic field energy fractions.
• Employs standard fireball model parameters: electron spectral index p = 2.4 ± 0.11, electron energy fraction ε_e,r = 0.6, magnetic field fraction ε_B,r = 0.125, and Lorentz factor γ_r = 1000.
• Derives the transition time from fast to slow cooling (t_tr,f ≈ 8.7 s) to explain the onset of the smooth tail at ~14 s after trigger.
• Considers pair production (γγ → e⁺e⁻) as a possible cause for the flux drop at ~14 s, due to interaction between prompt and forward shock photons.

Experimental results

Research questions

• RQ1 Can SSC emission from the reverse shock in a thick-shell, magnetized fireball explain the 2-second hard gamma-ray afterglow in GRB 980923?
• RQ2 Does the observed spectral index of −1.44 ± 0.07 for the high-energy component match the predictions of the SSC model in the fast-cooling regime?
• RQ3 Can the observed flux of 21.2 × 10⁻⁶ erg/cm²/s and break energy of ~427.9 MeV be reproduced by SSC from the reverse shock under standard equipartition assumptions?
• RQ4 Is the timing of the high-energy component (starting ~14 s after trigger) consistent with the reverse shock dynamics and cooling evolution?
• RQ5 What magnetic field configuration and electron energy distribution are required to reproduce the observed afterglow features?

Key findings

• The SSC emission from the reverse shock produces a break energy ν(IC)_m ≈ 427.9 MeV, which matches the observed high-energy component extending up to 150 MeV.
• The calculated peak flux F(IC)_max ≈ 21.2 × 10⁻⁶ erg/cm²/s is consistent with the observed flux level of the hard component.
• The spectral index of the SSC component is −1.44 ± 0.07, matching the observed value and indicating a fast-cooling regime.
• The model predicts that the high-energy flare occurs during the prompt phase, consistent with the observed 2-second duration and onset at ~14 s.
• The reverse shock magnetic field is estimated to be B_r ≈ 0.125 G, significantly stronger than the forward shock field (B_f ≈ 0.9 × 10⁻³ B_r), supporting a magnetized ejecta origin.
• The transition from fast to slow cooling in the forward shock occurs at t_tr,f ≈ 8.7 s, consistent with the onset of the 400 s smooth tail at ~14 s after the trigger.