Pulsating Tail Emission of Magnetar Giant Flares with a Dipolar-multipolar Magnetic Field Configuration

Abstract Magnetar giant flares (GFs) consist of a bright initial spike and a pulsating tail that persists for hundreds of seconds. Observed pulsating tail light curves feature multi-peaked morphologies, large amplitude ratios, and almost stable pulse phases. A fireball formed in a pure dipole or multipolar magnetic field is insufficient to explain the observed complex light curve features. In this paper, we study the radiation of a fireball trapped in a magnetar with a dipolar-multipolar magnetic field configuration, involving the energy injection from the magnetar’s crust. We obtain the structure and evolution of the trapped fireball. Combined with the magnetar rotation, the magnetic field configuration, and the viewing angle, the light curves of the pulsating tail emission are obtained. It is shown that the obtained light curves are consistent with the characteristics of the pulsating tail emission of GFs—not only reproducing the multi-peaked morphology, large peak-trough amplitude ratio, and stable pulse phase observed in actual events, but also matching the individual differences in peak number of different magnetar GFs. This reveals that a composite dipole-multipolar magnetic field configuration is essential for explaining GFs.