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

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.

11 Figures, 2 Tables, accepted for publication in ApJ