From Rare Events to a Population: Discovering Overlooked Extragalactic Magnetar Giant Flare Candidates in Archival Fermi Gamma-ray Burst Monitor Data

TL;DR

This work enlarges the extragalactic magnetar giant flare (MGF) sample by identifying four new candidates in Fermi/GBM archival data, bringing the total to 13 MGFs and enabling population-level inference.A forward-folding, multi-instrument MCMC framework is developed to jointly model IPN and GBM detections, incorporating spectral diversity through an $E_p$–$E_{ m iso}$ relation and GBM sensitivity, to recover intrinsic MGF energetics and rates.The analysis finds a power-law isotropic-energy distribution with slope $\beta\approx 1.7$, a relatively high minimum energy $E_{ m iso,min}$, and a volumetric rate around $\sim 10^{5}$–$10^{6}$ Gpc$^{-3}$ yr$^{-1}$ at 30 Mpc, implying that magnetars flare repeatedly over their lifetimes.Recurrence is required by the data (roughly several flares per magnetar), and while MGFs are common in star-forming galaxies, only a small fraction are expected to accompany FRB emission, limiting MGFs as a primary FRB progenitor channel.

Abstract

Magnetar giant flares (MGFs) are rare, extremely bright bursts of gamma-rays from highly magnetized neutron stars. These events are challenging to identify because, at extragalactic distances, they can appear similar to other astrophysical phenomena. Only a handful have been confidently identified to date, limiting our understanding of their origin and physical properties. This study focuses on expanding the sample of known events and enabling a more detailed characterization of their observational features and intrinsic properties, while introducing significant improvements in the methods used to identify and analyze them. When applied to archival data from the Gamma-ray Burst Monitor (GBM) on the \Fermi Gamma-ray Space Telescope, this approach added four previously unidentified events the known sample, expanding the total to 13 MGFs. This demonstrates both the effectiveness of the method and the likelihood that additional MGFs remain hidden in existing gamma-ray burst catalogs. We utilize this expanded sample to gain a deeper understanding of the broader population of MGFs. We develop a statistical modeling framework that combines previously considered data with modern observations from Fermi/GBM. The model accounts for instrumental sensitivity and the expected diversity in event characteristics. We infer a volumetric rate of events above $1.2\times10^{44}\,\rm{erg}$ of $R_{MGF}=5.5^{+4.5}_{-2.7}\times10^5\rm{Gpc^{-3}yr^{-1}}$. The results show that individual magnetars must produce multiple flares throughout their lifetimes, reinforcing the idea that these are recurring phenomena rather than singular explosive events. Expanding the sample of known MGFs improves our understanding of magnetars and their role in other astrophysical phenomena, including possible links to fast radio bursts, gravitational waves, and the creation of heavy elements in extreme astrophysical environments.

Figures (6)

• Figure 1: Flowchart illustrating the refined selection criteria used to identify MGF candidates from the Fermi/GBM sGRB sample. Starting from 663 sGRBs with available TTE data, a series of temporal cuts is applied using BB analysis. GRBs with known redshift are excluded, yielding 68 MGF prospects.
• Figure 2: Cumulative $\Omega$ values for the 68 MGF candidates (black line) compared to the background distribution generated from random galaxy rotations. The shaded regions indicate the $1\sigma$ (dark blue) and $3\sigma$ (light blue) confidence intervals derived from resampling 10,000 background comparisons drawn from an initial set of 45,000 randomized galaxy realizations.
• Figure 3: Peak energy ($E_{\mathrm{p}}$) versus isotropic energy ($E_{\mathrm{iso}}$) for the sample of extragalactic MGF candidates and confirmed events. Each burst is labeled and color-coded. Error bars reflect uncertainties in both $E_{\mathrm{iso}}$ and $E_{\mathrm{p}}$. The black line shows the best-fit empirical relation. This relation is used to assign $E_{\mathrm{p}}$ values in the population model as a function of $E_{\mathrm{iso}}$.
• Figure 4: Normalized fluence scaling surface at 50–300 keV as a function of COMPT spectral parameters $\alpha$ and $E_{\mathrm{p}}$, constructed using a grid of model spectra and normalized to the observed fluence of GRB 200415A. Overlaid points correspond to observed MGF candidates. Units are relative and do not represent absolute fluence values.
• Figure 5: GBM detection probability as a function of 64 ms peak photon flux in the 50--300 keV band.