Magnetars are the most highly magnetized neutron stars in the cosmos (with magnetic field 1013–1015 G). Giant flares from magnetars are rare, short-duration (about 0.1 s) bursts of hard X-rays and soft γ rays1,2. Owing to the limited sensitivity and energy coverage of previous telescopes, no magnetar giant flare has been detected at gigaelectronvolt (GeV) energies. Here, we report the discovery of GeV emission from a magnetar giant flare on 15 April 2020 (refs. 3,4 and A. J. Castro-Tirado et al., manuscript in preparation). The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope detected GeV γ rays from 19 s until 284 s after the initial detection of a signal in the megaelectronvolt (MeV) band. Our analysis shows that these γ rays are spatially associated with the nearby (3.5 megaparsecs) Sculptor galaxy and are unlikely to originate from a cosmological γ-ray burst. Thus, we infer that the γ rays originated with the magnetar giant flare in Sculptor. We suggest that the GeV signal is generated by an ultra-relativistic outflow that first radiates the prompt MeV-band photons, and then deposits its energy far from the stellar magnetosphere. After a propagation delay, the outflow interacts with environmental gas and produces shock waves that accelerate electrons to very high energies; these electrons then emit GeV γ rays as optically thin synchrotron radiation. This observation implies that a relativistic outflow is associated with the magnetar giant flare, and suggests the possibility that magnetars can power some short γ-ray bursts.
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In this study we use the Fermi-LAT Science Tools, publicly available at the Fermi Science Support Center website: https://fermi.gsfc.nasa.gov/ssc/. The code required to reproduce each figure of the paper is available from the corresponding authors upon request.
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The Fermi-LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA and the Department of Energy (United States), CEA/Irfu and IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK and JAXA (Japan), and the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from INAF (Italy) and CNES (France) is also gratefully acknowledged. This work was performed in part under Department of Energy contract DE-AC02-76SF00515. E. Burns is a NASA Postdoctoral Program Fellow.
The authors declare no competing financial interests.
Peer review information Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.
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All fluxes are calculated in the 100 MeV to 10 GeV energy range.
We highlight those with high probability (>90%) to be associated with the LAT-detected source, according to the likelihood analysis. The uncertainty on the estimated γ-ray energies is of the order of 10%. The last two columns show the angular distance to the centre of NGC 253 (the Sculptor galaxy) and the 68% containment radius of the PSF.
LR values for 105 simulated ROIs using the standard Rayleigh formula (a) and using the TS map to compute the probability (b). The red distributions correspond to the point source hypothesis, while the blue distributions take into account of the galaxy extension. The step in the distributions at low LR is due to many low-LR trials occupying the first bin. The values of the LRs associated with the Sculptor galaxy are highlighted by red and blue vertical dashed lines for the two cases.
Summary of the probability for a random association with the Sculptor galaxy and with the GRB 200415A.
Distribution of the time intervals Δt for triplets formed by three consecutive photons with (green) and without (dashed red) taking into account the correction for the effects of the LAT orbit and FOV. The expected distribution in case of independent events is represented as a solid black line. The vertical line in blue shows the period of the Fermi orbit (5,790 s), while the yellow vertical line indicates Δt = 264.87 s corresponding to the photon triplet detected by the LAT after GBM detected emission from GRB 200415A.
Bayesian blocks representation of the arrival times of the γ rays with the prior parameter p = 3 (red). Light green and dark gray are the daily and weekly count rates, while the blue curve shows the weekly-averaged exposure (between 100 MeV and 300 GeV, assuming a power-law photon index of −2) for a 1°-radius ROI in the direction of Sculptor for the entire time of the mission. Values of the exposure, in units of 108 cm2 s, can be read from the right y-axis. The three yellow bands highlight three characteristic observing profiles: 35° rocking angle, at the beginning of the mission, an observation strategy favouring the Galactic Centre region, in the middle, and, lastly, the period between the start of the solar drive anomaly and the implementation of a reoptimized survey strategy.
a, Onset times (TLAT,0 ) in the 100 MeV–100 GeV band versus the end of the GRB as detected by GBM in the 50–300 keV energy range (TGBM,95 ). b, Durations (TLAT,100 ) calculated in the 100 MeV–100 GeV energy range versus the same quantities calculated in the 50–300 keV energy range (TGBM,90 ). The solid line denotes where the two values are equal. Empty Blue and filled red circles represent long and short GRBs, respectively (data from 2FLGC27]). GRB 200415A is added and marked with a yellow star. The two SGRBs 160702A and GRB 170127C from 2FLGC, which exhibit similar durations, are highlighted with a magenta circle and green square, respectively.
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The Fermi-LAT Collaboration. High-energy emission from a magnetar giant flare in the Sculptor galaxy. Nat Astron 5, 385–391 (2021). https://doi.org/10.1038/s41550-020-01287-8