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Broadband X-ray burst spectroscopy of the fast-radio-burst-emitting Galactic magnetar

Abstract

Magnetars are young, magnetically powered neutron stars that possess the strongest magnetic fields in the Universe. Fast radio bursts (FRBs) are extremely intense millisecond-long radio pulses of primarily extragalactic origin, and a leading attribution for their genesis focuses on magnetars. A hallmark signature of magnetars is their emission of bright, hard X-ray bursts of sub-second duration. On 27 April 2020, the Galactic magnetar SGR J1935+2154 emitted hundreds of X-ray bursts within a few hours. One of these temporally coincided with an FRB, the first known detection of an FRB from the Milky Way. Here, we present spectral and temporal analyses of 24 X-ray bursts emitted 13 hours prior to the FRB and seen simultaneously with the Neutron Star Interior Composition Explorer (NICER) mission of the National Aeronautics and Space Administration and with the Fermi Gamma-ray Burst Monitor (GBM) mission in their combined energy range of 0.2 keV to 30 MeV. These broadband spectra permit direct comparison with the spectrum of the FRB-associated X-ray burst (FRB-X). We demonstrate that all 24 NICER and GBM bursts are very similar temporally to the FRB-X, but strikingly different spectrally. The singularity of the FRB-X burst is perhaps indicative of an uncommon locale for its origin. We suggest that this event originated in quasi-polar open or closed magnetic field lines that extend to high altitudes.

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Fig. 1: Example of light curve and spectrum of one of the 24 bursts observed simultaneously with Fermi GBM and NICER.
Fig. 2: Spectral parameter distributions for all 24 bursts in our sample and comparison to the FRB-X.
Fig. 3: Cut-off energy versus flux in the 1–250 keV range for the 24 bursts in our sample.

Data availability

NICER raw data and cleaned Level 2 data files were generated at the Goddard Space Flight Center large-scale facility. These data files, with observation ID 3020560101, can be found at https://heasarc.gsfc.nasa.gov/FTP/nicer/data/obs/2020_04/3020560101. Fermi GBM data files were generated at the Marshall Space Flight Center large-scale facility, and can be found at https://heasarc.gsfc.nasa.gov/FTP/fermi/data/gbm/daily/2020/04/28/current. Level 3 data that support the findings of this study are available from the corresponding authors upon request. Source data are provided with this paper.

Code availability

Reduction and analysis of the data were conducted using publicly available codes provided by the High Energy Astrophysics Science Archive Research Center, which is a service of the Astrophysics Science Division at the NASA Goddard Space Flight Center and of the High Energy Astrophysics Division of the Smithsonian Astrophysical Observatory. For NICER, NICERDAS version 7a, which is part of HEASoft 6.27.2 (https://heasarc.gsfc.nasa.gov/docs/software/lheasoft), was used. Fermi tools version 1.0.3 and GSpec version 0.9.1 were used for the analysis of Fermi GBM data (https://fermi.gsfc.nasa.gov/ssc/data/analysis/gbm/). Spectral analysis was conducted using Xspec version 12.11.0 (https://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/). Custom codes used to create plots presented in this manuscript are available from the corresponding authors upon request. These used Python libraries NumPy42, SciPy43 and Matplotlib44. Source data are provided with this paper.

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Acknowledgements

A portion of this work was supported by the National Aeronautics and Space Administration (NASA) through the NICER mission and the Astrophysics Explorers Program. This research has made use of data and software provided by the High Energy Astrophysics Science Archive Research Center, which is a service of the Astrophysics Science Division at the NASA Goddard Space Flight Center and of the High Energy Astrophysics Division at the Smithsonian Astrophysical Observatory. G.Y. acknowledges support from NASA under NICER Guest Observer Cycle 1 program 2098 (grant no. 80NSSC19K1452). C.K. acknowledges support from NASA under Fermi Guest Observer Cycle 10 (grant no. 80NSSC17K0761). M.G.B. acknowledges the generous support from the National Science Foundation (grant no. AST-1813649). T.E. is supported by the Japan Society for the Promotion of Science and the Ministry of Education, Culture, Sports, Science and Technology (KAKENHI grant nos. 18H01246 and 18H04584) and by the RIKEN Hakubi project. S.G. acknowledges support from the French National Centre for Space Studies. W.C.G.H. acknowledges support from NASA (grant no. 80NSSC20K0278). NICER research at the United States Naval Research Laboratory is supported by NASA.

Author information

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Authors

Contributions

G.Y. performed the data reduction and analysis and contributed to the writing of the manuscript. M.G.B. contributed to the theoretical interpretation of the results and to the writing of the manuscript. C.K. contributed to the interpretation of the results and to the writing of the manuscript. Z.A. is the NICER project scientist; he contributed to the scheduling of the NICER observation, discussion on specific data analysis related to deadtime, and discussion and editing of the paper. T.E. is the chair of the NICER magnetar and magnetosphere science group and is responsible for observation planning of magnetars with NICER. J.D. designed the NICER electronics and contributed to the discussion related to data analysis and deadtime in the NICER data. K.C.G. is the NICER principal investigator; he approved the Director’s Discretionary Time observation that led to the detection of the burst storm with NICER. E.G., S.G., T.G., A.K.H., W.C.G.H., A.J.v.d.H., C.-P.H., G.K.J., Y.K., L.L., P.S.R., O.J.R., W.M. and Z.W. contributed to the discussion and editing of the manuscript. B.J.L. and J.F.S. contributed to the discussion related to deadtime in NICER. T.O. is the NICER optics lead. J.P. and M.S. are part of the NICER operations team that worked to get NICER on SGR J1935+2154 as quickly as possible.

Corresponding authors

Correspondence to G. Younes, M. G. Baring or C. Kouveliotou.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Burst durations and spectral parameters.

Time is from 2020 April 28, 00 h. The burst with an asterisk is the one burst where a BB+CPL model is required to provide a statistically good fit to the data. A 2BB model cannot provide a good fit as is evident from the last column. The burst highlighted in bold face is the one presented in Fig. 1 of the main text. Numbers in parentheses represent the 1s uncertainty on the corresponding last digit.

Source data

Extended Data Fig. 2 T90 distribution of the 24 bursts in our sample.

The blue bar represents the T90 of the FRB-associated burst as measured with HXMT13.

Extended Data Fig. 3 Distributions of the spectral parameters of a CPL model that best fit 10000 simulated NICER+GBM spectra.

The simulated spectra are drawn from the best fit CPL model to the HXMT FRB-associated burst13.

Source data

Source Data Extended Data Fig. 1

Machine-readable version of Extended Data Fig. 1.

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Younes, G., Baring, M.G., Kouveliotou, C. et al. Broadband X-ray burst spectroscopy of the fast-radio-burst-emitting Galactic magnetar. Nat Astron 5, 408–413 (2021). https://doi.org/10.1038/s41550-020-01292-x

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