2025ApJ...994...10B

The mechanism responsible for the prompt gamma-ray emission of a gamma-ray burst (GRB) continues to remain an enigma. The detailed analysis of the spectrum of GRB 131014A observed by the Fermi Gamma-ray Burst Monitor and Large Area Telescope has revealed an unconventional spectral shape that significantly deviates from the typical Band function. The spectrum exhibits three distinctive breaks and an extended power law at higher energies. Furthermore, the lower end of the spectrum aligns with power-law indices greater than -0.5, and in the brightest region of the burst, these values approach +1. The lowest spectral break is thereby found to be consistent with a blackbody. These observed spectral characteristics strongly suggest the radiation process to be inverse Compton scattering (ICS) in an optically thin region. Applying the empirical fit parameters for physical modeling, we find that the kinetic energy of the GRB jet of bulk Lorentz factor, Γ ∼ 400, gets dissipated just above the photosphere, approximately at a radius of ∼10¹⁴ cm. The electrons involved in this process are accelerated to a power-law index of δ = -1.5, and the minimum electron Lorentz factor, ${\gamma }_{\min }$, is approximately 3. In summary, this study provides a comprehensive identification and detailed modeling of optically thin ICS in the prompt emission of GRB 131014A.