2025ApJ...986..189T

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 the absence of the X-ray shallow decay for hyperenergetic GRBs, recently discovered GRB 240529A shows a clear shallow decay phase with an isotropic gamma-ray energy of 2.2 × 10⁵⁴ erg, 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-Large Area Telescope (LAT) analysis of GRB 240529A along with the Swift-X-Ray Telescope 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 hyperenergetic GRBs with a typical shallow decay phase. The GeV to keV flux ratio is calculated to be 4.2 ± 2.3. Together with the 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 the Cherenkov Telescope Array Observatory, will distinguish between them.