Fermi-GBM Observations of the SGR J1935+2154 Burst Forest

TL;DR

This study analyzes the spectral evolution of a 130-second burst forest from magnetar SGR J1935+2154 observed by Fermi/GBM, revealing thermal components, pulse modulation, and a hotspot origin within magnetic field lines.

Contribution

First detailed time-resolved spectral analysis of a magnetar burst forest, linking spectral evolution to geometrical and magnetic field configurations.

Findings

Thermal spectral components dominate throughout the burst forest.

High-energy maxima align with flux peaks and are modulated by the spin period.

The hot component's emitting area is much smaller, suggesting a low-altitude hotspot.

Abstract

During 2020 April and May, SGR J1935+2154 emitted hundreds of short bursts and became one of the most prolific transient magnetars. At the onset of the active bursting period, a 130 s burst "forest," which included some bursts with peculiar time profiles, were observed with the Fermi/Gamma-ray Burst Monitor (GBM). In this Letter, we present the results of time-resolved spectral analysis of this burst "forest" episode, which occurred on 2020 April 27. We identify thermal spectral components prevalent during the entire 130 s episode; high-energy maxima appear during the photon flux peaks, which are modulated by the spin period of the source. Moreover, the evolution of the $\nu F_{\nu}$ spectral hardness (represented by $E_{\rm peak}$ or blackbody temperature) within the lightcurve peaks is anti-correlated with the pulse phases extrapolated from the pulsation observed within the persistent soft X-ray emission of the source six hours later. Throughout the episode, the emitting area of the high-energy (hotter) component is 1-2 orders of magnitude smaller than that for the low-energy component. We interpret this with a geometrical viewing angle scenario, inferring that the high-energy component likely originates from a low-altitude hotspot located within closed toroidal magnetic field lines.