A bright gamma-ray flare interpreted as a giant magnetar flare in NGC 253

On 15 April 2020 the extremely bright, short gamma-ray burst GRB 200415A was detected by five space-based missions of the Interplanetary network of gamma-ray detectors (IPN). We report here the final 20 arcmin2 burst localization by the IPN, which overlaps the central part of the nearby galaxy NGC 253, at a distance of 3.5 Mpc. The chance occurrence for GRB 200415A to be spatially consistent with a nearby galaxy likely to produce detectable giant flares is approximately 1 in 200000. As observed by Konus Wind, the burst light curve starts with the fast, (2 ms) rise of a narrow (4 ms) initial spike, which is followed by an exponentially decaying phase with count-rate e-folding time 50 ms . The total burst duration is 0.138 s. The burst is strikingly similar to GRB 051103, that presumably originated from the M81/M82 group of galaxies at nearly the same distance. The characteristic radius of the emission region, estimated from the blackbody spectral fits, is 20-40 km, of the same order as the radius of a neutron star or its magnetosphere. The implied isotropic-equivalent energies released in gamma-rays, Eiso, are 1.3 and 5.3×10⁴⁶ erg, and the isotropic-equivalent peak luminosities, Liso, are 1.4 and 1.8x 10⁴⁸ erg/s, comparable to those of a galactic giant magnetar flare.