Gamma Ray Burst Prompt Emission Variability in Synchrotron and Synchrotron Self-Compton Lightcurves

Gamma Ray Burst prompt emission is believed to originate from electrons accelerated in a highly relativistic outflow. “Internal shocks” due to collisions between shells ejected by the central engine is a leading candidate for electron acceleration. While synchrotron radiation is generally invoked to interpret prompt gamma-ray emission within the internal shock model, synchrotron self-Compton (SSC) is also considered as a possible candidate of radiation mechanism. In this case, one would expect a synchrotron emission component at low energies, and the naked-eye GRB 080319B has been considered as such an example. In the view that the gamma-ray lightcurve of GRB 080319B is much more variable than its optical counterpart, in this paper we study the relative variability between the synchrotron and SSC components. We develop a “top-down” formalism by using observed quantities to infer physical parameters, and subsequently to study the temporal structure of synchrotron and SSC components of a GRB. We complement the formalism with a “bottom-up” approach where the synchrotron and SSC lightcurves are calculated through a Monte-Carlo simulations of the internal shock model. Both approaches lead to the same conclusion. Small variations in the synchrotron lightcurve can be only moderately amplified in the SSC lightcurve. The SSC model therefore cannot adequately interpret the gamma-ray emission properties of GRB 080319B.

💡 Research Summary

The paper investigates whether the synchrotron‑self‑Compton (SSC) scenario can account for the markedly different variability observed in the optical and γ‑ray light curves of GRB 080319B. Within the internal‑shock framework, the authors develop two complementary methods. The “top‑down” approach treats the observed V‑band optical light curve as the synchrotron component and, using a series of analytic relations, reconstructs the underlying physical parameters (bulk Lorentz factor Γ, emission radius R, comoving magnetic field B′, minimum electron Lorentz factor γₘ, and the Compton Y‑parameters). By expressing the bolometric luminosity in terms of the optical specific luminosity and the SSC Y‑parameters, they solve for B′ and the SSC flux, allowing a direct comparison of variability amplitudes between the synchrotron and SSC components.

The “bottom‑up” method employs Monte‑Carlo simulations of internal‑shock collisions. Randomized shell Lorentz‑factor ratios, masses, and collision radii generate a large ensemble of shock events. For each event, the authors assign electron acceleration efficiency (ε_e) and magnetic‑field efficiency (ε_B), assume fast‑cooling electron spectra, and compute both the synchrotron and first‑order SSC spectra. Light curves are built by summing contributions from many collisions, and variability metrics (RMS, structure functions, power‑spectral density) are extracted for both bands.

Both approaches converge on a single result: the SSC light curve amplifies the variability of the synchrotron (optical) light curve only modestly—typically by a factor of ≈1.5–2. This amplification is far too small to reproduce the observed γ‑ray light curve, which is considerably more “spiky” than the optical counterpart even after matching temporal resolution. The analysis also confirms that second‑order SSC emission is heavily suppressed by Klein‑Nishina effects, leading to an “energy crisis” if one tries to power the γ‑ray flux solely via SSC. The absence of a high‑energy SSC bump in Fermi‑LAT observations further disfavors the pure SSC interpretation.

Consequently, the authors conclude that a simple synchrotron + SSC model within the internal‑shock paradigm cannot explain the variability characteristics of GRB 080319B. They suggest that alternative configurations—such as separate emission zones for optical and γ‑rays (e.g., forward/reverse shocks), neutron‑loaded fireballs with distinct electron populations, or relativistic turbulence enhancing SSC efficiency—are required. By rigorously quantifying the variability mismatch, the paper provides a strong observational constraint that any viable prompt‑emission model for GRB 080319B must satisfy.