Shear Particle Acceleration in Structured Gamma-Ray Burst Jets. IV. Thermal versus Nonthermal Emission of the Jet Cocoon

A distinct thermal or quasi-thermal spectral component is occasionally observed in gamma-ray burst (GRB) prompt emission spectra. Taking GRB 090902B as a case study, we investigate its origin within a structured jet framework, in which the outflow consists of an ultrarelativistic uniform core surrounded by a structured cocoon. In the weak-scattering regime with inefficient shear acceleration, electrons pre-energized in the thin jet–cocoon interaction layer are further heated in the mixed jet–cocoon (MJC) region, forming a quasi-thermal electron distribution. Parameterizing the radial temperature profile of electrons as a power law with index q T , we demonstrate that both the peak flux and spectral width of the thermal component are sensitive to maximum temperature T max and q T . Combined with the synchrotron emission of shock-accelerated electrons in the jet core, our model reproduces both the quasi-thermal component in the keV–MeV range and the broadband nonthermal emission observed in the time-integrated and time-resolved spectra of GRB 090902B. A comparative analysis of GRB 240825A within a shear-acceleration dominated (strong-scattering) scenario shows that shear-accelerated electrons produce broader spectra than thermalized electrons in the weak-scattering regime. These results indicate that GRB spectral diversity likely arises from the additional emission component originating in the MJC region under different physical conditions.