High-Energy Emission from a Magnetar Giant Flare in the Sculptor Galaxy

Magnetars are the most highly-magnetized neutron stars in the cosmos. Giant flares from magnetars are rare, short-duration bursts of hard X-rays and soft gamma rays. In this talk, we report the discovery of GeV emission from a magnetar giant flare (MGF) on April 15, 2020. The Fermi Large Area Telescope (LAT) detected GeV gamma rays from 19 seconds until 284 seconds after the initial detection of a signal in the MeV band. Our analysis shows that these gamma rays are spatially associated with the nearby (3.5 Mpc) Sculptor galaxy and are unlikely to originate from a cosmological gamma-ray burst. We infer that the gamma rays originated with the MGF in Sculptor and we suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons.