Bright γ-ray flares observed in GRB131108A

by Gudlaugur Johannesson (Science Institute, University of Iceland)

Wednesday 27 May 2020, 13:30 → 14:30 Europe/Stockholm

https://stockholmuniversity.zoom.us/j/530682073

https://arxiv.org/abs/1911.04642

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 γ-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 γ-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 ∼ 2.0) × 10−6 erg cm−2, is comparable to that of emission during the steep decay phase, 1.7 × 10−6 erg cm−2. The total fluence from three γ-ray flares is 5.3 × 10−6 erg cm−2. The three γ-ray flares show properties similar to the usual X-ray flares that are sharp flux increases, occurring in ∼ 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 (α^ = 2 + β^), which is the first observational evidence of the high-latitude emission in the GeV energy band.