Speaker
Description
The MeV afterglow in gamma-ray bursts (GRBs) represents a largely unexplored window into the transition from prompt emission to forward-shock-dominated afterglow. At MeV energies, the emission could arise from synchrotron radiation of the decelerating forward shock or from lingering prompt emission components. Each of these mechanisms carries distinct signatures of jet physics, magnetic field structure, and central engine behaviour. Therefore, disentangling these mechanisms is critical, yet capturing the MeV emission from slowly varying transients after the prompt emission (after T90) is challenging as MeV instruments are typically background-dominated. While Swift-BAT is sensitive to early afterglow emission, it detects only a fraction (typically 35%) of GRBs observed by Fermi-GBM. The broader detection rate and hard X-ray coverage of Fermi-GBM make it a powerful tool for probing MeV emission beyond T90. In our work, we applied a novel method of extracting the MeV afterglow from GBM-detected bursts using orbital background subtraction. This allows us to capture the faint MeV emission from GRBs beyond the prompt emission phase, which cannot be recovered via conventional analysis techniques. Applying this technique to a sample of the ~20 most fluent GRBs from 18 years of Fermi-GBM observations, we systematically characterise the spectral and temporal properties. Tracking the spectral index evolution across our sample reveals changing radiative processes that govern the prompt-to-afterglow transition.
In my presentation, I’ll discuss these findings, which also have direct implications for TeV studies of GRBs, as understanding the MeV afterglow and particle acceleration processes provides crucial constraints on the origin and evolution of very-high-energy (TeV) emission.
