Magnetar giant flare originating from GRB 200415A: transient GeV emission, time-resolved E_p – L _iso correlation and implications

Giant flares (GFs) are unusual bursts from soft gamma-ray repeaters (SGRs) that release an enormous amount of energy in a fraction second. The afterglow emission these SGR-GFs or GF candidates is highly beneficial means discerning their composition, relativistic speed, and mechanisms. GRB 200415A recent candidate observed direction coincident with the nearby Sculptor galaxy at 3.5 Mpc. In this work, we searched for transient past observations by Fermi-LAT 200415A. These confirm as GeV source only once. A pure pair-plasma fireball cannot provide required interpretation baryonic poor outflow additionally needed to explain emission. rich also viable, it can variability quasi-thermal spectrum prompt if dissipation happening below photosphere via internal shocks. Using peak ($E_p$) time-resolved spectra fluxes ($F_p$), found correlation between $E_p$ $F_p$ isotropic luminosity $L_{\rm iso}$ This supports intrinsic nature $E_p$-$E_{\rm SGRs-GFs, hence favoring outflow. Our results indicate different mechanism work during initial spike, evolution spectral properties would be intrinsically due injection process.