Fermi Gamma-ray Space Telescope

Investigating The Nature of High-Energy Emission from GRBs Through Joint Fermi/Swift Observations

D. Kocevski
J. Racusin

Abstract:

We present a systematic investigation into the nature of the delayed and long-lived high-energy emission observed from gamma-ray bursts by the Fermi Large Area Telescope (LAT). The origin of this emission has been much debated within the GRB community, and has led to speculation that the afterglow component may produce a significant amount of gamma-ray emission due the high-energy extension of the synchrotron spectrum produced by the GRB blast wave. Using Swift-XRT observations of Fermi-LAT detected bursts, we find that the LAT detected population exhibit X-ray afterglows that are among the brightest and hardest detected by Swift. Analysis of simultaneous XRT and LAT observations reveals that a majority of these bright LAT detected bursts require a break in their broadband spectrum between the XRT and LAT energy ranges. Such a break is expected to persist in the afterglow spectrum at high energies to very late times if the GRB blast wave is propagating into a wind like circumburst medium. This effectively allows the afterglow to remain bright at gamma-ray wavelengths to very late times, enabling their detection by the Fermi-LAT. Therefore, we propose that a majority of GRBs detected at late times by Fermi-LAT may preferentially probe environments in which their progenitor underwent sustained mass loss prior to the core collapse event that triggered the GRB.