Soft γ-ray repeaters exhibit bursting emission in hard X-rays and soft γ-rays. During the active phase, they emit random short (milliseconds to several seconds long), hard-X-ray bursts, with peak luminosities1 of 1036 to 1043 erg per second. Occasionally, a giant flare with an energy of around 1044 to 1046 erg is emitted2. These phenomena are thought to arise from neutron stars with extremely high magnetic fields (1014 to 1015 gauss), called magnetars1,3,4. A portion of the second-long initial pulse of a giant flare in some respects mimics short γ-ray bursts5,6, which have recently been identified as resulting from the merger of two neutron stars accompanied by gravitational-wave emission7. Two γ-ray bursts, GRB 051103 and GRB 070201, have been associated with giant flares2,8,9,10,11. Here we report observations of the γ-ray burst GRB 200415A, which we localized to a 20-square-arcmin region of the starburst galaxy NGC 253, located about 3.5 million parsecs away. The burst had a sharp, millisecond-scale hard spectrum in the initial pulse, which was followed by steady fading and softening over 0.2 seconds. The energy released (roughly 1.3 × 1046 erg) is similar to that of the superflare5,12,13 from the Galactic soft γ-ray repeater SGR 1806−20 (roughly 2.3 × 1046 erg). We argue that GRB 200415A is a giant flare from a magnetar in NGC 253.
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The Fermi (https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/triggers/2020/bn200415367/current/), Swift (https://www.swift.psu.edu/guano/) and INTEGRAL (http://isdc.unige.ch/~savchenk/spiacs-online/spiacs.pl) data are freely available online. The HEND data used for the triangulation and Konus–Wind lightcurve and spectral data are available at http://www.ioffe.ru/LEA/papers/SvinkinNat2020/data/. Links to the Wind ephemeris and clock accuracy data are provided in Methods. Source data are provided with this paper.
XSPEC is freely available online (https://heasarc.gsfc.nasa.gov/xanadu/xspec/).
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We thank E. Burns for discussions, O. Roberts for reading the manuscript and providing comments, and V. Pal’shin for contributing to the Konus–Wind and IPN data analysis tools. A.B., P.U. and J.C.R. acknowledge the continuous support from the Italian Space Agency ASI via different agreements including the latest one, 2019-35-HH.0. The Konus–Wind experiment is supported by the Russian State Space Corporation ROSCOSMOS. The HEND experiment is supported by ROSCOSMOS and implemented as part of Gamma-Ray Spectrometer suite on NASA Mars Odyssey. HEND data processing is funded by Ministry of Science and Higher Education of the Russian Federation, grant AAAA-A18-118012290370-6.
The authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 GRB 051103 (red stars) and GRB 200415A (blue stars) as possible cosmological GRBs at various redshifts (0.01 < z < 1).
The Konus–Wind samples of short–hard GRBs and long GRBs with known redshifts25 are shown by green triangles and grey circles, respectively. The recent update25 for the hardness–intensity relation in the cosmological rest frame (Ep,z − Eiso, ‘Amati’ relation) is plotted as a solid line, together with its 68% and 90% prediction intervals (dashed black lines). Considering only its spectrum and energy fluence, GRB 200415A is consistent with the Konus–Wind sample of short GRBs if at redshift z ≈ 0.05−1. In the case of GRB 051103, the implied short-GRB redshift is z ≈ 1, with intrinsic Ep ≈ 5 MeV.
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Svinkin, D., Frederiks, D., Hurley, K. et al. A bright γ-ray flare interpreted as a giant magnetar flare in NGC 253. Nature 589, 211–213 (2021). https://doi.org/10.1038/s41586-020-03076-9
Nature Astronomy (2021)