Constraining Gamma-Ray Burst Viewing Angles with Swift/XRT Afterglow Light Curves
5 affiliations
- 1Qufu Normal University
- 2Huazhong University of Science and Technology
- 3Chinese Academy of Sciences
- 4University of Science and Technology of China
- 5Nanjing University
Abstract
Gamma-ray bursts (GRBs) are among the most energetic phenomena in the Universe, and their afterglow light curves provide insights into jet geometry and viewing angle. The interaction between relativistic ejecta and the circumburst medium produces afterglow emission, and jet-break features offer crucial information about viewing geometry. To constrain GRB viewing angles by analyzing jet-break features in the Swift X-Ray Telescope afterglow light curves, we constructed and compared two models: a simplified geometric model without high-latitude emission (model 1) and a comprehensive model with high-latitude emission (model 2). Employing these two models, we examined viewing angles and off-axis ratios ( q = θ obs / θ jet ) under different density profiles, and evaluated the impact of high-latitude emission on our modeling approach. Both models were applied to a selected sample of 20 GRBs in the interstellar medium (ISM) and 20 GRBs in the wind medium, with criteria ensuring that the jet breaks were attributed to the edge effect and that data coverage was sufficient. Markov Chain Monte Carlo methods were employed for parameter estimation. Model comparison based on the reduced chi-squared ( χ red 2 ) and the Bayesian information criterion (BIC) indicates that model 1 provides a better fit to the observed light curves for all GRBs in the sample. We find that most GRBs in the sample have small off-axis ratios ( q ¯ = 0.1851 for model 1), suggesting that the viewing angles are generally close to the jet axis; the distribution of viewing angles in logarithmic space follows an approximately Gaussian pattern. A Kolmogorov–Smirnov test reveals no significant difference in the distribution of off-axis ratios between the ISM and wind medium. We find no significant difference in the off-axis ratio between bursts with and without an X-ray plateau. While the viewing angle decreases significantly with redshift, the off-axis ratio shows no significant evolution, consistent with the degree of off-axis alignment being independent of cosmic epoch.
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BibTeX
@article{sun2026constraining,
title = {{Constraining Gamma-Ray Burst Viewing Angles with Swift/XRT Afterglow Light Curves}},
author = {Cheng-Jie Sun and Shuang-Xi Yi and Lin Zhou and Yuan-Chuan Zou and Yu-Peng Yang and Si-Ji Xin and Yan-Kun Qu and Wen-Long Zhang and Fa-Yin Wang},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/ae9f57},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/ae9f57},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - Constraining Gamma-Ray Burst Viewing Angles with Swift/XRT Afterglow Light Curves
AU - Cheng-Jie Sun
AU - Shuang-Xi Yi
AU - Lin Zhou
AU - Yuan-Chuan Zou
AU - Yu-Peng Yang
AU - Si-Ji Xin
AU - Yan-Kun Qu
AU - Wen-Long Zhang
AU - Fa-Yin Wang
PY - 2026
DA - 2026/10/15
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/ae9f57
UR - https://iopscience.iop.org/article/10.3847/1538-4357/ae9f57
AB - Gamma-ray bursts (GRBs) are among the most energetic phenomena in the Universe, and their afterglow light curves provide insights into jet geometry and viewing angle. The interaction between relativistic ejecta and the circumburst medium produces afterglow emission, and jet-break features offer crucial information about viewing geometry. To constrain GRB viewing angles by analyzing jet-break features in the Swift X-Ray Telescope afterglow light curves, we constructed and compared two models: a simplified geometric model without high-latitude emission (model 1) and a comprehensive model with high-latitude emission (model 2). Employing these two models, we examined viewing angles and off-axis ratios ( q = θ obs / θ jet ) under different density profiles, and evaluated the impact of high-latitude emission on our modeling approach. Both models were applied to a selected sample of 20 GRBs in the interstellar medium (ISM) and 20 GRBs in the wind medium, with criteria ensuring that the jet breaks were attributed to the edge effect and that data coverage was sufficient. Markov Chain Monte Carlo methods were employed for parameter estimation. Model comparison based on the reduced chi-squared ( χ red 2 ) and the Bayesian information criterion (BIC) indicates that model 1 provides a better fit to the observed light curves for all GRBs in the sample. We find that most GRBs in the sample have small off-axis ratios ( q ¯ = 0.1851 for model 1), suggesting that the viewing angles are generally close to the jet axis; the distribution of viewing angles in logarithmic space follows an approximately Gaussian pattern. A Kolmogorov–Smirnov test reveals no significant difference in the distribution of off-axis ratios between the ISM and wind medium. We find no significant difference in the off-axis ratio between bursts with and without an X-ray plateau. While the viewing angle decreases significantly with redshift, the off-axis ratio shows no significant evolution, consistent with the degree of off-axis alignment being independent of cosmic epoch.
SN - 0004-637X
SN - 1538-4357
ER -