GRB 231129C: Another Thermal Emission Dominated Gamma-Ray Burst

This study presents detailed time-integrated and time-resolved spectral analysis of the Fermi Gamma-ray Burst Monitor observations of the bright GRB 231129C. The results reveal its distinct spectral characteristics, featuring a hard low-energy spectral index (alpha) and soft high-energy spectral index (beta), similar to GRB 090902B, suggesting a possible dominance of thermal emission. Further analysis indicates that 92% of the spectral indices exceed the synchrotron "line of death," with the hardest index at alpha similar to +0.44. Simultaneously, 53% of the spectra can be well fitted by the nondissipative photosphere model, supporting a potential origin from a nondissipative photosphere. Additionally, we observe strong correlations between the spectral index alpha and peak energy E p with flux. For the alpha-F relationship, we employ F = F 0 e (3.00 +/- 0.10)alpha to describe it, whereas the E p-F relationship requires a smoothly bending power-law function. Based on the framework proposed by Hasco & euml;t et al. and Gao & Zhang, the jet characteristics of this burst were studied, revealing that both methods support the suitability of a pure fireball model for this GRB at small initial jet radii.