Magnetar giant flare in NGC 253 seen by Fermi-GBM

Magnetar giant flares (MGFs) are enormous eruptions likely triggered by surface disruptions in magnetars, neutron stars with the strongest-known magnetic fields. Such events can be detected in both X- and gamma-ray bands, but are very rare. Almost 30 magnetars have been cataloged in our Galaxy, exhibiting occasional X-ray activity, but only two have produced giant flares to date. The most recent one, emitted by SGR 1806-20 in 2004, showed an initial very short and bright main spike, causing the saturation of the observing instruments and thus precluding reliable flux measurements. Here we report the observation and analysis of GRB 200415A, a very short and bright Gamma-Ray Burst detected by the Fermi Gamma-Ray Burst Monitor (GBM) as well as by several other instruments participating in the InterPlanetary Network (IPN) system, which located it in a region spatially coincident with the nearby galaxy NGC 253. Analysis of the event revealed peculiar spectral and temporal properties, which are not typically seen in GRBs: a very short rise time of the initial hard spike, strong submillisecond variability, a flat spectrum, and an unusually low isotropic energy release. Therefore we concluded that GRB 200415A is not a classical short GRB due to the merger of two binary neutron stars, but rather a MGF produced by an extragalactic magnetar.