Magnetar Giant Flare Originated GRB 200415A: Transient GeV emission, Time-Resolved ${\mathrm{E}}_{\mathrm{p}}-{\mathrm{L}}_{\mathrm{iso}}$ Correlation, and Implications

Abstract

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

arXiv commentAccepted for publication in RAA

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