Rapid Spectral Variability of a Giant Flare from a Magnetar in NGC 253

TL;DR

This paper reports the detection of a giant flare from an extragalactic magnetar in NGC 253, exhibiting rapid spectral variability and relativistic plasma motion, providing insights into magnetar flare mechanisms and their gamma-ray signatures.

Contribution

It presents the first detailed observations of a giant flare from an extragalactic magnetar, including spectral evolution and relativistic effects, expanding understanding of such rare events.

Findings

Detection of rapid spectral variability in the flare

Evidence of relativistic plasma motion from 3 MeV photons

Association of the flare with galaxy NGC 253 at 3.5 Mpc

Abstract

Magnetars are slowly-rotating neutron stars with extremely strong magnetic fields ($10^{13-15}$ G), episodically emitting $\sim100$ ms long X-ray bursts with energies of $\sim10^{40-41}$ erg. Rarely, they produce extremely bright, energetic giant flares that begin with a short ($\sim0.2$ s), intense flash, followed by fainter, longer lasting emission modulated by the magnetar spin period (typically 2-12 s), thus confirming their origin. Over the last 40 years, only three such flares have been observed in our local group; they all suffered from instrumental saturation due to their extreme intensity. It has been proposed that extra-galactic giant flares likely constitute a subset of short gamma-ray bursts, noting that the sensitivity of current instrumentation prevents us from detecting the pulsating tail, while the initial bright flash is readily observable out to distances $\sim 10-20$ Mpc. Here, we report X- and gamma-ray observations of GRB 200415A, which exhibits a rapid onset, very fast time variability, flat spectra and significant sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extra-galactic magnetar, noting that GRB 200415A is directionally associated with the galaxy NGC 253 ($\sim$3.5 Mpc away). The detection of $\sim3$ MeV photons provides definitive evidence for relativistic motion of the emitting plasma. The observed rapid spectral evolution can naturally be generated by radiation emanating from such rapidly-moving gas in a rotating magnetar.