2025/04/15 by Kenta Terauchi, Tomohiko Oka, Terauchi, Kenta +3
Physics and Astronomy · #Astronomy and Astrophysical Research #Astrophysics and Cosmic Phenomena #FOS: Physical sciences #Gamma-ray bursts and supernovae #High Energy Astrophysical Phenomena (astro-ph.HE)
paper · pdf · doi:10.48550/arxiv.2504.10907
openalex publication_date 2025/04/15 · openalex created_date 2025/10/10 · openalex updated_date 2026/07/28
X-ray light curves of gamma-ray burst (GRB) afterglows exhibit various features, with the shallow decay phase being particularly puzzling. While some studies report absence of the X-ray shallow decay for hyper-energetic GRBs, recently discovered GRB 240529A shows a clear shallow decay phase with an isotropic gamma-ray energy of \SI2.2e54erg, making it a highly unusual case compared to typical GRBs. In order to investigate the physical mechanism of the shallow decay, we perform the Fermi-LAT analysis of GRB 240529A along with Swift-XRT analysis. We find no jet break feature in the X-ray light curve and then give the lower bound of the collimation-corrected jet energy of >1052~erg, which is close to the maximum rotational energy of a magnetar. Our LAT data analysis reveals evidence of GeV emission with a statistical significance of 4.5σ during the shallow decay phase, which can be interpreted as the first case for hyper-energetic GRBs with a typical shallow decay phase. The GeV to keV flux ratio is calculated to be 4.2±2.3. Together with X-ray spectral index, this indicates an inverse Compton origin of the GeV emission. Multiwavelength modeling based on time-dependent simulations tested two promising models, the energy injection and wind models. Both models can explain the X-ray and gamma-ray data, while our modeling demonstrates that gamma-ray observations, along with future GeV--TeV observations by CTAO, will distinguish between them.