External Inverse-Compton Emission Associated with Extended and Plateau Emission of Short Gamma-Ray Bursts: Application to GRB 160821B

The recent detection of TeV photons from two gamma-ray bursts (GRBs), GRB 190114C and GRB 180720B, has opened a new window for multimessenger and multiwavelength astrophysics of high-energy transients. We study the origin of very high energy (VHE) γ-rays from the short GRB 160821B, for which the MAGIC Collaboration reported a ∼3σ statistical significance. Short GRBs are often accompanied by extended and plateau emission, which is attributed to internal dissipation resulting from activities of a long-lasting central engine, and Murase et al. (2018) recently suggested the external inverse-Compton (EIC) scenario for VHE counterparts of short GRBs and neutron star mergers. Applying this scenario to GRB 160821B, we show that the EIC flux can reach ∼10^-12 erg cm^-2 s^-1 within a time period of ∼10³-10⁴ s, which is consistent with the MAGIC observations. EIC γ-rays expected during the extended and plateau emission will be detectable with greater significance by future detectors such as the Cerenkov Telescope Array. The resulting light curve has a distinguishable feature, where the VHE emission is predicted to reach the peak around the end of the seed photon emission.