Magnetar Giant Flare Origin for GRB 200415A Inferred from a New Scaling Relation

Soft gamma-ray repeaters (SGRs) are a mainly Galactic population and originate from neutron stars with intense (B ≃ 10¹⁵ G) magnetic fields (magnetars). Occasionally, a giant flare occurs with enormous intensity, displaying a short, hard spike followed by a weaker, oscillatory phase that exhibits the rotational period of the neutron star. If the magnetar giant flares occur in nearby galaxies, they would appear as cosmic, short-hard gamma-ray bursts (GRBs) without detecting the weak oscillatory phase. Recently, a short-hard GRB named GRB 200415A was detected, with a position coincident with the Sculptor Galaxy (NGC 253), raising the question of whether it is a classic short GRB or a magnetar giant flare. Here we show that magnetar giant flares follow a scaling relation between the spectral peak energy and the isotropic energy in 1 keV–10 MeV, i.e., $E_p\propto E_{iso}^{1/4}$, and locate in a distinct region of the E _p–E _iso plane from that of classic short GRBs. The relation can be well understood in the model that giant flares arise from the photosphere emissions of relativistically expanding fireballs. GRB 200415A, together with two other candidate giant flares (GRB 051103 and GRB 070201) follow this relation, which strongly favors the giant flare origin of these GRBs. The GeV emission detected by Fermi/LAT from GRB 200415A at 18–285 s can also be explained in the giant flare scenario. The total energy in the GeV emission implies a baryon load of ∼10²³ g in the giant flare fireball of GRB 200415A.