Understanding the delayed GeV emissions in Gamma Ray Bursts

Gamma-ray bursts (GRBs) are ultrarelativistic jetted outflows that carry observational signatures of the extreme environments surrounding the compact objects that produced them. These jets are formed during catastrophic events such as massive star collapses or compact binary mergers, generating highly variable prompt emissions and broadband afterglows. However, the exact nature of the radiative processes, including the origin of GeV photons during the decaying phase of GRB afterglows, remains an open question. While the typical power-law behavior of afterglow spectra is attributed to the synchrotron emission from the particles accelerated in the external shock of the jet, it can’t explain the detection of high-energy emissions at late times. As the shock decelerates on interacting with the medium, the particles in the jet get cooled enough, making them incapable of producing GeV photons through synchrotron alone. To address this gap in our understanding of GRB physics, we performed a detailed prompt-to-afterglow time-resolved spectral study of an exceptionally bright gamma-ray burst in keV-GeV band. In this presentation, I shall discuss the key evidence in our study that supports an additional emission mechanism giving rise to the rare GeV photons during the afterglow. We found a peculiar spectral hardening at times much later than the end of the prompt emission, hinting towards the existence of a second component in the GRB spectra. This shall serve as a stepping stone in understanding the peculiar rebrightenings in GRB spectra that are not expected by the conventional afterglow theory. I’ll present our results constraining the jet dynamics and microphysical properties governing the production of such interesting emissions. I’ll also discuss the possibility of a very high-energy counterpart associated with the given burst. This can provide a deep insight into how different emission mechanisms and microphysical properties of the shock interplay, giving rise to peculiar emissions in GRBs.