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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE
Abstract:
GRB 131108A is a bright long Gamma-Ray Burst (GRB) detected by the Large Area
Telescope and the Gamma-ray Burst Monitor on board the \textit{Fermi Gamma-ray
Space Telescope}. Dedicated temporal and spectral analyses reveal three
$\gamma$-ray flares dominating above 100 MeV, which are not directly related to
the prompt emission in the GBM band (10 keV--10 MeV). The high-energy light
curve of GRB 131108A (100 MeV -- 10 GeV) shows an unusual evolution: a steep
decay, followed by three flares with an underlying emission, and then a
long-lasting decay phase. The detailed analysis of the $\gamma$-ray flares
finds that the three flares are 6 -- 20 times brighter than the underlying
emission and are similar to each other. The fluence of each flare, (1.6 $\sim$
2.0) $\times$ 10$^{-6}$ erg cm$^{-2}$, is comparable to that of emission during
the steep decay phase, 1.7 $\times$ 10$^{-6}$ erg cm$^{-2}$. The total fluence
from three $\gamma$-ray flares is 5.3 $\times$ 10$^{-6}$ erg cm$^{-2}$. The
three $\gamma$-ray flares show properties similar to the usual X-ray flares
that are sharp flux increases, occurring in $\sim$ 50\% of afterglows, in some
cases well after the prompt emission. Also, the temporal and spectral indices
during the early steep decay phase and the decaying phase of each flare show
the consistency with a relation of the curvature effect ($\hat{\alpha}$ = 2 +
$\hat{\beta}$), which is the first observational evidence of the high-latitude
emission in the GeV energy band.
