High-energy spectral cutoffs in the prompt emission of Fermi gamma-ray bursts: bulk Lorentz factors, emission radii, and a cutoff-peak energy relation
Abstract
The high-energy end of the gamma-ray burst (GRB) prompt spectrum carries information on the physical conditions of the relativistic outflow, but the number of well-characterized spectral cutoffs is small. We present a systematic search for high-energy spectral cutoffs in the time-integrated prompt spectra of 139 GRBs observed by Fermi between 2008 July and 2025 December, selected to have broadband coverage either through a LAT detection or through bright BGO emission. Joint GBM, LLE, and LAT spectra over T90 are fitted with four empirical models and compared using the Bayesian information criterion, yielding 106 bursts with well-constrained cutoff energies Ec. An empirical two-component Gaussian mixture model applied to log10(Ec/MeV) separates a low-Ec group from the main distribution at Ec=2.12 MeV; the 77 bursts lying firmly above this boundary are interpreted within the internal γγ pair-opacity framework. Combining Ec with wavelet-based minimum variability timescales (median $0.119$ s), we infer bulk Lorentz factors of $11.7$ to 2.78×103 (median 145) and emission radii of ∼1013 to 1015 cm. Neither the Γ-Liso nor the Γ-Eiso relation is statistically significant for the full sample, whereas the 27 bursts with measured redshifts show a moderate Γ-Liso correlation with a slope of 0.36−0.23+0.22. We further find a strong rest-frame correlation between the cutoff and peak energies, Ec,z∝ Ep,z2.15−0.97+0.96, for the 26 bursts with measured redshifts, which persists after controlling for redshift.
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BibTeX
@misc{zuo2026highenergy,
title = {{High-energy spectral cutoffs in the prompt emission of Fermi gamma-ray bursts: bulk Lorentz factors, emission radii, and a cutoff-peak energy relation}},
author = {Yuan-Yuan Zuo and Yuan-Chuan Zou},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.14411},
eprint = {2609.14411},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.14411},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - High-energy spectral cutoffs in the prompt emission of Fermi gamma-ray bursts: bulk Lorentz factors, emission radii, and a cutoff-peak energy relation
AU - Yuan-Yuan Zuo
AU - Yuan-Chuan Zou
PY - 2026
DA - 2026/09/13
PB - arXiv
DO - 10.48550/arxiv.2609.14411
UR - https://arxiv.org/abs/2609.14411
AB - The high-energy end of the gamma-ray burst (GRB) prompt spectrum carries information on the physical conditions of the relativistic outflow, but the number of well-characterized spectral cutoffs is small. We present a systematic search for high-energy spectral cutoffs in the time-integrated prompt spectra of 139 GRBs observed by Fermi between 2008 July and 2025 December, selected to have broadband coverage either through a LAT detection or through bright BGO emission. Joint GBM, LLE, and LAT spectra over T_90 are fitted with four empirical models and compared using the Bayesian information criterion, yielding 106 bursts with well-constrained cutoff energies E_c. An empirical two-component Gaussian mixture model applied to log_10(E_c/MeV) separates a low-E_c group from the main distribution at E_c=2.12 MeV; the 77 bursts lying firmly above this boundary are interpreted within the internal γγ pair-opacity framework. Combining E_c with wavelet-based minimum variability timescales (median $0.119$ s), we infer bulk Lorentz factors of $11.7$ to 2.78×10^3 (median 145) and emission radii of ∼10^13 to 10^15 cm. Neither the Γ-L_iso nor the Γ-E_iso relation is statistically significant for the full sample, whereas the 27 bursts with measured redshifts show a moderate Γ-L_iso correlation with a slope of 0.36_−0.23^+0.22. We further find a strong rest-frame correlation between the cutoff and peak energies, E_c,z∝ E_p,z^2.15_−0.97^+0.96, for the 26 bursts with measured redshifts, which persists after controlling for redshift.
ER -