Searching for an additional high-energy component in Fermi-LAT GRB afterglows

Abstract

Context. The very high-energy (VHE; ≥100 GeV) component from at least two gamma-ray bursts (GRBs), that is, GRB 180720B and GRB 190114C, has been detected in the afterglow phase. It is widely discussed that the GeV to TeV emission originated from a synchrotron self-Compton (SSC) process. The VHE component may cause an upturn at the high-energy spectral ends in the Fermi-Large Area Telescope (Fermi-LAT) observing band. Aims. We aim to find out whether an additional high-energy component commonly exists in the afterglows of Fermi-LAT GRBs. This study will help us to better understand how common it is for a GRB afterglow detected by Fermi-LAT to involve a VHE component. Methods. First, we selected the GRBs that emit ≥10 GeV photons. The ≥10 GeV photons can be considered as a plausible proxy for a VHE component. We systematically analyzed 199 GRBs detected by Fermi-LAT from 2008–2019. If an additional high-energy component exists in the afterglows of Fermi-LAT GRBs, the best-fit spectral model could be a broken power law (BPL) model with an upturn above a break energy. We compared the afterglow spectra using power-law (PL) and BPL representations. Results. Out of the 30 GRBs with ≥10 GeV photons that arrived after T90 (the time duration when 90% of the prompt emission was detected), 25 GRBs are tentatively or significantly detected at 0.1–200 GeV after 2 × T90. The spectrum of GRB 131231A shows an upturn above an energy break of 1.6 ± 0.8 GeV, supporting the BPL model. For GRB 131231A, we performed a modeling of its X-ray and γ-ray spectra and found that the SSC model can explain the upturn with acceptable parameter values. In the cases of GRB 190114C, GRB 171210A, GRB 150902A, GRB 130907A, GRB 130427A, and GRB 090902B, the improvement of the BPL fit compared to the PL fit is tentative or marginal. Conclusions. There is no conclusive evidence that an additional higher energy component commonly exists in Fermi-LAT GRB afterglows, except for the group of Fermi-LAT GRBs mentioned above. Such an additional high-energy component may be explained by the SSC mechanism. Current and future VHE observations will provide important constraints on the issue.