Speaker
Description
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 4.5σ 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.
