Abstract
The Fermi-LAT collaboration presented the second gamma-ray burst (GRB)
catalog covering its first 10 years of operations. A significant fraction of
afterglow-phase light curves in this catalog cannot be explained by the closure
relations of the standard synchrotron forward-shock model, suggesting that
there could be an important contribution from another process. In view of the
above, we derive the synchrotron self-Compton (SSC) light curves from the
reverse shock in the thick- and thin-shell regime for a uniform-density medium.
We show that this emission could explain the GeV flares exhibited in some LAT
light curves. Additionally, we demonstrate that the passage of the forward
shock synchrotron cooling break through the LAT band from jets expanding in a
uniform-density environment may be responsible for the late time ($\approx10^2$
s) steepening of LAT GRB afterglow light curves. As a particular case, we model
the LAT light curve of GRB 160509A that exhibited a GeV flare together with a
break in the long-lasting emission, and also two very high energy photons with
energies of 51.9 and 41.5 GeV observed 76.5 and 242 s after the onset of the
burst, respectively. Constraining the microphysical parameters and the
circumburst density from the afterglow observations, we show that the GeV flare
is consistent with a SSC reverse-shock model, the break in the long-lasting
emission with the passage of the synchrotron cooling break through the
Fermi-LAT band and the very energetic photons with SSC emission from the
forward shock when the outflow carries a significant magnetic field ($R_{\rm B}
\simeq 30$) and it decelerates in a uniform-density medium with a very low
density ($n=4.554^{+1.128}_{-1.121}\times 10^{-4}\,{\rm cm^{-3}}$).