Magnetars are neutron stars with extremely strong magnetic fields (1013 to 1015 gauss)1,2, which episodically emit X-ray bursts approximately 100 milliseconds long and with energies of 1040 to 1041 erg. Occasionally, they also produce extremely bright and energetic giant flares, which begin with a short (roughly 0.2 seconds), intense flash, followed by fainter, longer-lasting emission that is modulated by the spin period of the magnetar3,4 (typically 2 to 12 seconds). Over the past 40 years, only three such flares have been observed in our local group of galaxies3,4,5,6, and in all cases the extreme intensity of the flares caused the detectors to saturate. It has been proposed that extragalactic giant flares are probably a subset7,8,9,10,11 of short γ-ray bursts, given that the sensitivity of current instrumentation prevents us from detecting the pulsating tail, whereas the initial bright flash is readily observable out to distances of around 10 to 20 million parsecs. Here we report X-ray and γ-ray observations of the γ-ray burst GRB 200415A, which has a rapid onset, very fast time variability, flat spectra and substantial sub-millisecond spectral evolution. These attributes match well with those expected for a giant flare from an extragalactic magnetar12, given that GRB 200415A is directionally associated13 with the galaxy NGC 253 (roughly 3.5 million parsecs away). The detection of three-megaelectronvolt photons provides evidence for the relativistic motion of the emitting plasma. Radiation from such rapidly moving gas around a rotating magnetar may have generated the rapid spectral evolution that we observe.
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γ-ray data from CGRO–BATSE, Swift–BAT and Fermi–GBM are available in public repositories on NASA’s High Energy Astrophysics Science Archive Research Center (HEASARC; https://heasarc.gsfc.nasa.gov/w3browse/all/batsegrb.html, https://heasarc.gsfc.nasa.gov/W3Browse/swift/swiftgrb.html and https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html, respectively); catalogues of these data are provided as citations. The raw VLA data are publicly available (https://archive.nrao.edu). The calibrated VLA data and images are available from the corresponding authors on reasonable request.
Standard software packages, such as rmfit for GBM and XSPEC for other instruments, are available online (https://fermi.gsfc.nasa.gov/ssc/data/analysis/rmfit/ and https://heasarc.gsfc.nasa.gov/docs/software.html, respectively). The codes used to determine the significance of the BGO photons, to construct the BAT TTE detector response matrices and to determine the rise time are available from the corresponding authors on reasonable request. The VLA data were analysed using publicly available software (CASA). The procedure for detecting and quantifying the QPOs is publicly available in Stingray (https://stingray.readthedocs.io/en/latest/). The algorithm used to determine the pulse pile-up of the GBM data is available in ref. 35. The SwiMM code is not publicly available; however, response functions can be used to reproduce our spectral results; these are available from the corresponding authors on reasonable request.
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The Fermi GBM Collaboration acknowledges the support of NASA in the United States under grant NNM11AA01A and of DRL in Germany. P.V. acknowledges support from NASA grant 80NSSC19K0595. A.T. and J.J.D. thank T. Sakamoto for access to the Swift mass model. We thank the National Radio Astronomy Observatory, a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities. D.H. acknowledges support from the DIRAC Institute in the Department of Astronomy at the University of Washington. The DIRAC Institute is supported through gifts from the Charles and Lisa Simonyi Fund for Arts and Sciences and the Washington Research Foundation. J.J.D. acknowledges that this material is based on work supported by the National Science Foundation under grants PHY-1708146 and PHY-1806854 and by the Institute for Gravitation and the Cosmos of Pennsylvania State University.
The authors declare no competing interests.
Peer review information Nature thanks the anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
The T90 (green) and T50 (purple) durations were calculated using the Swift–BAT data in count space. The errors are at the 1σ confidence level.
The νFν spectra (top) and Comptonized fitting residuals (bottom) are shown for intervals (1) (left), (3) (centre) and (4) (right) of GRB 200415A. The three spectra are devoid of any instrumental effects attributed to bandwidth saturation. The fit parameters are listed in Table 1. These figures show the robustness of the fits to the data (1σ confidence), which are used in the main text and in Fig. 1d, and are a direct result of the unrivalled temporal and spectral quality of the GBM data. Arrows on the error bars are due to the lower or upper limits being unconstrained. The solid blue lines are the best fits to the data.
The grey histogram represents the counts with energies of 0.2–40 MeV (left axis). Individual TTEs of GBM BGO detector 0 are shown as black circles, superimposed over the grey histogram, with photon energies in MeV (right axis). The blue rectangle indicates energies of 2.5–3.5 MeV in intervals (2) and (3); the red rectangle shows energies of 3.5–10 MeV. We conclude that the highest photon energy unambiguously associated with GRB 200415A is 3 MeV.
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Roberts, O.J., Veres, P., Baring, M.G. et al. Rapid spectral variability of a giant flare from a magnetar in NGC 253. Nature 589, 207–210 (2021). https://doi.org/10.1038/s41586-020-03077-8
Nature Astronomy (2021)