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A magnetar giant flare in the nearby starburst galaxy M82


Magnetar giant flares are rare explosive events releasing up to 1047 erg in gamma rays in less than 1 second from young neutron stars with magnetic fields up to 1015−16 G (refs. 1,2). Only three such flares have been seen from magnetars in our Galaxy3,4 and in the Large Magellanic Cloud5 in roughly 50 years. This small sample can be enlarged by the discovery of extragalactic events, as for a fraction of a second giant flares reach luminosities above 1046 erg s−1, which makes them visible up to a few tens of megaparsecs. However, at these distances they are difficult to distinguish from short gamma-ray bursts (GRBs); much more distant and energetic (1050−53 erg) events, originating in compact binary mergers6. A few short GRBs have been proposed7,8,9,10,11, with different amounts of confidence, as candidate giant magnetar flares in nearby galaxies. Here we report observations of GRB 231115A, positionally coincident with the starburst galaxy M82 (ref. 12). Its spectral properties, along with the length of the burst, the limits on its X-ray and optical counterparts obtained within a few hours, and the lack of a gravitational wave signal, unambiguously qualify this burst as a giant flare from a magnetar in M82.

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Fig. 1: Optical image of M82.
Fig. 2: Light curves of GRB 231115A.
Fig. 3: Comparison between GRB 231115A and the 2004 giant flare from SGR 1806–20.

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Data availability

The data of the INTEGRAL, XMM-Newton and Swift satellites are publicly available in the respective online archives (,, Optical data are available upon request.

Code availability

The software used for the data analysis is public and can be retrieved at,,


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We thank the ESA Mission Scientists J.-U. Ness and N. Schartel for approving and quickly implementing the INTEGRAL and XMM-Newton ToO observations. This work is based on observations with INTEGRAL and XMM-Newton, ESA missions with instruments and science data centres funded by ESA member states, and with the participation of the Russian Federation and the United States. It is also based on observations made with the Italian TNG operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This paper includes optical data taken with the Schmidt 67/92 telescope operated by INAF Osservatorio Astronomico di Padova (Mount Ekar, Asiago). This work received financial support from INAF through the Magnetars Large Program Grant (Principal Investigator S.M.) and from the GRAWITA Large Program Grant (Principal Investigator P.D.A.). J.C.R., A.B., S.M. and P.U. acknowledge financial support from ASI under contract no. 2019-35-HH.0. F.O. acknowledges support from MIUR, PRIN 2020 (grant no. 2020KB33TP) ‘Multimessenger astronomy in the Einstein Telescope Era’ (METE). J.C.R. acknowledges support from the European Union’s Horizon 2020 Programme under the AHEAD2020 project (grant agreement no. 871158). P.D.A. and S.C. acknowledge funding from the Italian Space Agency, contract ASI/INAF no. I/004/11/4.

Author information

Authors and Affiliations



All authors reviewed the manuscript and contributed to the source interpretation. S.M. coordinated the work and the interpretation of the results, contributed to the analysis of the INTEGRAL and XMM-Newton data, and wrote most of the manuscript. R.S. and E.A. contributed to write the main part of the paper. D.P.P. and J.C.R. carried out most of the INTEGRAL data analysis. D.G., C.F., E.B., L.D. and V.S. routinely contribute to the operation of the IBAS software and participated to the near real time INTEGRAL analysis. P.D.A. coordinated the analysis of the optical data from Italian telescopes. M.R. analysed the XMM-Newton data and contributed to the INTEGRAL spectral analysis. S.C. analysed the Swift data. M.T. contributed to the software for the burst search in archival data. D.T., W.T., D.S. and C.A. coordinated the observation and the analysis of the optical data taken at OHP. L.T. analysed the optical data taken with the Schmidt 67/92 telescope in Asiago under the Large Program ‘Search and characterization of optical counterparts of GW triggers’ (P.I. Tomasella). A.R. and E.C. triggered, reduced and analysed the observations at the Asiago Schmidt telescope. R.B. and M.F. provided the short GRB afterglows and kilonovae observed and simulated optical light curves.

Corresponding author

Correspondence to Sandro Mereghetti.

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Nature thanks Eric Burns and David Palmer for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 EPIC-pn images of M82.

The exposure-corrected images refer to the 0.3−2 keV (a) and 2−10 keV (b) energy ranges. The 90% c.l. error circle of GRB 231115A has a radius of 2 arcmin.

Extended Data Fig. 2 Maps of count rate upper limits.

The figure gives 3-σ upper limits on the 2−10 keV count rates of the EPIC-MOS1 (a), EPIC-MOS2 (b) and EPIC-pn (c) cameras. The 90% c.l. error circle of GRB 231115A has a radius of 2 arcmin.

Extended Data Fig. 3 Maps of flux upper limits.

The figure gives 3-σ upper limits on the fluxes in the 2−10 keV (a) and 0.3−10 keV (b) energy range, obtained by combining the three maps of Extended Data Fig. 2. The 90% c.l. error circle of GRB 231115A has a radius of 2 arcmin.

Extended Data Fig. 4 X-ray light curves of short GRB afterglows.

The Swift/XRT (black square) and XMM-Newton/EPIC (black diamond) 3-σ upper limits of GRB 231115A are indicated.

Extended Data Fig. 5 Optical light curves of kilonovae.

The r-band light curves of AT2017gro and of the faintest red kilonova (simulated with the POSSIS code) are shown with dashed and solid lines, respectively, assuming the M82 distance (3.6 Mpc). The magnitude 3-σ upper limits obtained for a position inside (outside) the M82 galaxy are shown as blue (red) arrows.

Extended Data Fig. 6 Optical light curves of short GRB afterglows.

The observed light curves are shown in a, while b shows the light curves of those GRBs which have a measure of redshift, rescaled to the M82 distance (3.6 Mpc). The 3-σ upper limits obtained for a position inside (outside) the M82 galaxy are shown as blue (red) arrows.

Extended Data Table 1 Results of spectral fits of GRB 231115A
Extended Data Table 2 Log of optical observations of GRB 231115A

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Mereghetti, S., Rigoselli, M., Salvaterra, R. et al. A magnetar giant flare in the nearby starburst galaxy M82. Nature 629, 58–61 (2024).

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