  Magnetars are young, highly magnetized neutron stars that produce extremely
rare giant flares of gamma-rays, the most luminous astrophysical phenomena in
our Galaxy. The detection of these flares from outside the Local Group of
galaxies has been predicted, with just two candidates so far. Here we report on
the extremely bright gamma-ray flare GRB 200415A of April 15, 2020, which we
localize, using the Interplanetary Network, to a tiny (20 sq. arcmin) area on
the celestial sphere, that overlaps the central region of the Sculptor galaxy
at 3.5 Mpc from the Milky Way. From the Konus-Wind detections, we find a
striking similarity between GRB 200415A and GRB 051103, the even more energetic
flare that presumably originated from the M81/M82 group of galaxies at nearly
the same distance (3.6 Mpc). Both bursts display a sharp, millisecond-scale,
hard-spectrum initial pulse, followed by an approximately 0.2 s long steadily
fading and softening tail. Apart from the huge initial pulses of magnetar giant
flares, no astrophysical signal with this combination of temporal and spectral
properties and implied energy has been reported previously. At the inferred
distances, the energy released in both flares is on par with that of the
December 27, 2004 superflare from the Galactic magnetar SGR 1806-20, but with a
higher peak luminosity. Taken all together, this makes GRB 200415A and its twin
GRB 051103 the most significant candidates for extragalactic magnetar giant
flares, both a factor of five more luminous than the brightest Galactic
magnetar flare observed previously, thus providing an important step towards a
better understanding of this fascinating phenomenon.
