The Evolution of Thermal and Non-thermal Emission Components in GRB 250920C

@misc{chen2026evolution,
  title = {{The Evolution of Thermal and Non-thermal Emission Components in GRB 250920C}},
  author = {Jia-Ming Chen and Ke-Rui Zhu and Shan Chang and Dan Zhu and Yu-Lu Gong and Zhao-Yang Peng and Yong-Gang Zheng and Li Zhang},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2609.09710},
  eprint = {2609.09710},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2609.09710},
}

TY  - UNPB
TI  - The Evolution of Thermal and Non-thermal Emission Components in GRB 250920C
AU  - Jia-Ming Chen
AU  - Ke-Rui Zhu
AU  - Shan Chang
AU  - Dan Zhu
AU  - Yu-Lu Gong
AU  - Zhao-Yang Peng
AU  - Yong-Gang Zheng
AU  - Li Zhang
PY  - 2026
DA  - 2026/09/09
PB  - arXiv
DO  - 10.48550/arxiv.2609.09710
UR  - https://arxiv.org/abs/2609.09710
AB  - We present a temporal and time-resolved spectral analysis of GRB 250920C using \textit{Fermi}/GBM and \textit{Swift}/BAT data. The prompt emission consists of two distinct episodes, EI and EII, separated by a significant quiescent interval. We perform Bayesian spectral fitting with empirical, thermal, composite, and physical synchrotron models. In EI, the spectra show clear evidence for an additional thermal component. The BB+PL model is preferred in most bright time bins, and joint \textit{Fermi}/GBM+\textit{Swift}/BAT fits further support the presence of this component. The blackbody temperature generally decreases with time, the blackbody flux follows the pulse profile, and the thermal flux fraction remains high. Fireball-parameter estimates give a photospheric Lorentz factor of a few hundred and an initial radius of r_0∼10^9--10^10 cm, with 1+σ_0 of order unity and η≫1, supporting a thermally dominated baryonic outflow. In contrast, EII is dominated by non-thermal emission. Its low-energy photon indices do not significantly exceed the synchrotron line of death, and the spectra are well described by fast-cooling synchrotron radiation in a decaying magnetic field. The magnetization constraint gives σ_min∼1.1--$4.3$. Since these values exceed unity, they suggest that EII may be Poynting-flux dominated. These results therefore suggest a possible transition in GRB 250920C from a fireball-dominated EI phase to a Poynting-flux-dominated EII phase.
ER  -