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Magnetars are the most highly magnetized neutron stars in the cosmos (with magnetic field 10 13 –10 15 G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft γ rays 1 , 2 . Owing to the limited sensitivity and energy coverage of previous telescopes, no magnetar giant flare has been detected at gigaelectronvolt (GeV) energies. Here, we report the discovery of GeV emission from a magnetar giant flare on 15 April 2020 (refs. 3 , 4 and A. J. Castro-Tirado et al., manuscript in preparation). The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV γ rays from 19 s until 284 s after the initial detection of a signal in the megaelectronvolt (MeV) band. Our analysis shows that these γ rays are spatially associated with the nearby (3.5 megaparsecs) Sculptor galaxy and are unlikely to originate from a cosmological γ-ray burst. Thus, we infer that the γ rays originated with the magnetar giant flare in Sculptor. We suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas and produces shock waves that accelerate electrons to very high energies; these electrons then emit GeV γ rays as optically thin synchrotron radiation. This observation implies that a relativistic outflow is associated with the magnetar giant flare, and suggests the possibility that magnetars can power some short γ-ray bursts. Gigaelectronvolt emission from a magnetar giant flare is discovered by the Fermi Gamma-ray Space Telescope, between 19 s and 284 s after the initial detection of a signal in the megaelectronvolt energy band, potentially generated by an ultra-relativistic outflow far from the stellar magnetosphere.

Relativistic jets around supermassive black holes (SMBHs) are well-known powerfulγ -ray emitters. In absence of the jets in radio-quiet active galactic nuclei (AGNs), how the SMBHs work inγ -ray bands is still unknown despite of great observational efforts made in the last 3 decades. Considering the previous efforts, we carefully select an AGN sample composed of 37 nearby Seyfert galaxies with ultra-hard X-rays for the goals ofγ -ray detections by excluding all potential contamination in this band. Adopting a stacking technique, here we report the significantγ -ray detection ( \\rm TS=30.6 , or5.2 σ ) from the sample using 15-year Fermi-Large Area Telescope (LAT) observation. We find an averageγ -ray luminosity of the sample as(1.5±1.0)×10⁴⁰ \\rm erg s⁻¹at energies from 1-300 GeV. Limited by the well-known pair production from the interaction ofγ -rays with low energy photons,≳several GeVγ -rays are found to originate from an extended corona ( ∼ 2.7× 10⁶ R_(\\rm g) ), whereas the canonical much more compact X-ray corona ( ∼ 10 R_(\\rm g) ) is responsible for 1 to several GeVγ -rays. The finding of the compact region lends to strong supports to the long-time theoretical expectations, but the extended corona is beyond all the existing models. One promising scenario is that the electron-positron pairs produced in the compact X-ray corona would expand as fireball, similar to that inγ -ray bursts, forming the structure of extended corona.