From Empirical to Physical Model: Direct Fits of Optically Thin Inverse Compton Scattering to Prompt GRB Spectra

TL;DR

This study applies direct inverse Compton scattering models to GRB prompt emission spectra, constraining physical parameters and demonstrating its viability for explaining observed phenomena in certain bright GRBs.

Contribution

It introduces a physically consistent IC model fitting approach to GRB spectra, providing constraints on seed photons, electron populations, and dissipation environments.

Findings

Only 4 out of 41 bursts fit the IC criteria consistently.

Inferred seed photon energies are 0.05-0.2 keV, electron energies 20-300 keV.

Moderate optical depths and dissipation radii above the photosphere.

Abstract

Gamma-ray burst (GRB) prompt emission is commonly attributed to non-thermal radiation processes operating in the optically thin regions of a relativistic outflow. Among these, optically thin inverse-Compton (IC) scattering remains an important yet under-tested mechanism. From an initial set of 41 bursts selected using empirical Band-function criteria that highlight quasi-thermal low-energy slopes ($\alpha > -0.5$) and constrained high-energy indices ($-1.7 > \beta > -3.3$), only four events satisfy these conditions consistently in both time-integrated and time-resolved spectra. The IC fits yield self-consistent constraints on the seed-photon field and the electron population at the dissipation site. For bulk Lorentz factors $\Gamma \sim 170$-$550$, we infer seed thermal peaks of $\sim 0.05$-$0.2$ keV and electron thermal energies of $\sim 20$-$300$ keV in the co-moving frame. A fraction of only $0.1\%$-$20\%$ of electrons are accelerated into a non-thermal tail with an average index value of $\delta \sim 1.8$. The derived Comptonisation parameters indicate moderate $y$ values ($\sim 1$-$3$), optical depths $\tau \sim 0.2$-$0.6$, and dissipation radii just above the photosphere, consistent with mildly relativistic ($\gamma_{\min} \sim 1.2$-$2.6$), photon-dominated, low-magnetic-field dissipation environments. Furthermore, the framework allows us to constrain even sub-dominant thermal components that lie below the detector's low-energy threshold. Taken together, our results show that optically thin IC scattering offers a physically consistent and observationally viable explanation for the prompt emission in a subset of bright GRBs, motivating the application of IC models in future GRB studies.