2022MNRAS.517.3881L

Giant flares are the brightest and rarest outbursts from magnetars, with isotropic energies of 10⁴⁴-10⁴⁶ erg. GRB 200415A is suggested as a magnetar giant flare from NGC 253. Fermi Large Area Telescope detected the GeV afterglow emission from the flare, which is the first time that the GeV emission is detected from a giant flare. In this paper, we study the multiwavelength afterglow radiation of electrons accelerated by the forward and reverse shocks, produced via interactions between the ejecta and the circumburst medium in the afterglow phase of GRB 200415A. We found that in the GeV band, the forward shock emission is usually higher than the reverse shock emission, and can explain observations well, while in the optical and infrared bands, contributions from the forward and reverse shocks can be comparable for reasonable parameter sets, and the brightness of the forward-shock emission can reach 20 mag (AB). We predict that future telescopes such as Wide Field Survey Telescope, Chinese Space Station Telescope, the Large Synoptic Survey Telescope, and James Webb Space Telescope can detect the optical and infrared afterglow emission from giant flares similar to GRB 200415A. In addition, we consider two cases where the ejecta are dominated by protons or electron-positron pairs. We find that the reverse shock emission is comparable in these two cases for a hard electron spectrum, while for a soft electron spectrum, the reverse shock emission is much weaker in the pair-dominating case..