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We use joint observations by the Neil Gehrels Swift X-ray Telescope (XRT) andthe Fermi Large Area Telescope (LAT) of gamma-ray burst (GRB) afterglows toinvestigate the nature of the long-lived high-energy emission observed by FermiLAT. Joint broadband spectral modeling of XRT and LAT data reveal that LATnondetections of bright X-ray afterglows are consistent with a cooling break inthe inferred electron synchrotron spectrum below the LAT and/or XRT energyranges. Such a break is sufficient to suppress the high-energy emission so asto be below the LAT detection threshold. By contrast, LAT-detected bursts arebest fit by a synchrotron spectrum with a cooling break that lies eitherbetween or above the XRT and LAT energy ranges. We speculate that the primarydifference between GRBs with LAT afterglow detections and the non-detectedpopulation may be in the type of circumstellar environment in which thesebursts occur, with late-time LAT detections preferentially selecting GRBs thatoccur in low wind-like circumburst density profiles. Furthermore, we find noevidence of high-energy emission in the LAT-detected population significantlyin excess of the flux expected from the electron synchrotron spectrum fit tothe observed X-ray emission. The lack of excess emission at high energies couldbe due to a shocked external medium in which the energy density in the magneticfield is stronger than or comparable to that of the relativistic electronsbehind the shock, precluding the production of a dominant synchrotronself-Compton (SSC) component in the LAT energy range. Alternatively, the peakof the SSC emission could be beyond the 0.1-100 GeV energy range considered forthis analysis.

Investigating the Nature of Late-Time High-Energy GRB Emission Through Joint Fermi\Swift Observations