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 $2.2\times10^{54}$ 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-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 $>\!10^{52}$ erg, which is close to the maximum rotational energy of a magnetar.
Our LAT data analysis reveals GeV emission with a statistical significance of $4.5\sigma$ during the shallow decay phase, which is the first time for hyper-energetic GRBs with a typical shallow decay phase.
The GeV to keV flux ratio is calculated to be $4.2\pm2.3$. 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.
While the energy injection model shows a tension with LAT data, both models can explain the X-ray and GeV data.
We present our results along with the future prospects of the current or next generation gamma-ray telescopes for distinguishing between the shallow decay models.
For full details of the results, see the published paper \citep{Terauchi_2025}.