Fast radio bursts are millisecond-duration, extragalactic radio flashes of unknown physical origin1,2,3. The only known repeating fast radio burst source4,5,6—FRB 121102—has been localized to a star-forming region in a dwarf galaxy7,8,9 at redshift 0.193 and is spatially coincident with a compact, persistent radio source7,10. The origin of the bursts, the nature of the persistent source and the properties of the local environment are still unclear. Here we report observations of FRB 121102 that show almost 100 per cent linearly polarized emission at a very high and variable Faraday rotation measure in the source frame (varying from +1.46 × 105 radians per square metre to +1.33 × 105 radians per square metre at epochs separated by seven months) and narrow (below 30 microseconds) temporal structure. The large and variable rotation measure demonstrates that FRB 121102 is in an extreme and dynamic magneto-ionic environment, and the short durations of the bursts suggest a neutron star origin. Such large rotation measures have hitherto been observed11,12 only in the vicinities of massive black holes (larger than about 10,000 solar masses). Indeed, the properties of the persistent radio source are compatible with those of a low-luminosity, accreting massive black hole10. The bursts may therefore come from a neutron star in such an environment or could be explained by other models, such as a highly magnetized wind nebula13 or supernova remnant14 surrounding a young neutron star.
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We thank the staff of the Arecibo Observatory and the Green Bank Observatory for their help with our observations. We also thank B. Adebahr, L. Connor, G. Desvignes, R. Eatough, R. Fender, M. Haverkorn, A. Karastergiou, R. Morganti, E. Petroff, F. Vieyro and J. Weisberg for suggestions and comments on the manuscript. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968), and in alliance with Ana G. Méndez-Universidad Metropolitana and the Universities Space Research Association. The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Breakthrough Listen (BL) is managed by the Breakthrough Initiatives, sponsored by the Breakthrough Prize Foundation (http://www.breakthroughinitiatives.org). The research leading to these results received funding from the European Research Council (ERC) under the European Union’s Seventh Framework Programme (FP7/2007-2013). J.W.T.H. is a Netherlands Organisation for Scientific Research (NWO) Vidi Fellow and, together with D.M., K.G. and C.G.B., acknowledges funding for this work from ERC Starting Grant DRAGNET under contract number 337062. L.G.S. acknowledges financial support from the ERC Starting Grant BEACON under contract number 279702, as well as the Max Planck Society. A.M.A. is an NWO Veni Fellow. S.C., J.M.C., P.D., T.J.W.L., M.A.M. and S.M.R. are partially supported by the NANOGrav Physics Frontiers Center (NSF award 1430284). V.M.K. holds the Lorne Trottier Chair in Astrophysics & Cosmology and a Canada Research Chair and receives support from an NSERC Discovery Grant and Herzberg Prize, from an R. Howard Webster Foundation Fellowship from the Canadian Institute for Advanced Research (CIFAR), and from the FRQNT Centre de Recherche en Astrophysique du Québec. C.J.L. acknowledges support from NSF award 1611606. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. B.M. acknowledges support from the Spanish Ministerio de Economía y Competitividad (MINECO) under grants AYA2016-76012-C3-1-P and MDM-2014-0369 of ICCUB (Unidad de Excelencia ‘María de Maeztu’). S.M.R. is a CIFAR Senior Fellow. P.S. holds a Covington Fellowship at DRAO.
The authors declare no competing financial interests.
Reviewer Information Nature thanks H. Falcke and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
Extended Data Figure 1 Pulse profiles and spectra of 16 Arecibo bursts.
The bursts are dedispersed to DM = 559.7 pc cm−3 (which minimizes the width of burst 6) and plotted with time and frequency resolutions of 20.48 μs and 6.24 MHz, respectively.
Extended Data Figure 2 Polarimetric properties of the 11 brightest bursts detected by Arecibo.
a, Linear polarization fraction of the bursts as a function of frequency. The solid line shows the theoretical depolarization due to intra-channel Faraday rotation, calculated using equations (3) and (4). b, PA∞ as a function of frequency. Values in a and b are averaged over 16 consecutive channels. c, PA∞ as a function of time. A time offset is applied to each burst in order to show them consecutively. Vertical dashed lines divide different observing sessions. All values in this figure have been corrected for the rotation measure, which was calculated with a global fit. Grey regions in b and c indicate the 1σ uncertainty around the polarization angle determined from the global fit.
Extended Data Figure 3 Linear polarization fraction of the bursts as a function of rotation measure.
Different colours represent different observing sessions (see key). The grey line indicates the average rotation measure that yields the largest polarization fraction in the first observing session.
Extended Data Figure 4 Example of RM Synthesis and RMCLEAN results for burst 8.
The relevant rotation measure range is shown for burst 8, after analysis with RM Synthesis (dashed line) and RMCLEAN (solid line), as described in the main text. Only two clean components (red circles) were required to reach convergence in the deconvolution algorithm (at 102,679.5 rad m−2 and 102,679.75 rad m−2; compare with the peak of the final deconvolved Faraday spectrum at 102,679.65 rad m−2). For all bursts, the RM Synthesis and RMCLEAN steps demonstrate an extremely thin and single-peaked Faraday spectrum.
Extended Data Figure 5 Rotation measure and PA∞ values of different bursts.
Coloured, 1σ error bars represent individual bursts, with central values highlighted by black dots. Horizontal grey regions are values obtained from a global fit. MJD, modified Julian date. Values used in the figure are reported in Table 1.
Extended Data Figure 6 Physical constraints from source parameters.
a–c, Parameter space for the electron density (ne) and length scale (LRM) of the Faraday region for three different temperature regimes, Te = 104 K (a), 106 K (b) and 108 K (c). The shaded red region indicates the parameter space excluded because of optical depth considerations (optical depth from free–free absorption τff > 5). The solid black line indicates the maximum DMhost permitted, while the shaded grey region shows the dispersion measure down to 1 pc cm−3. The solid blue line denotes RMsrc. The shaded blue region shows the range 10−4 ≤ β ≤ 1. The intersection of grey and blue regions outside of the red region is physically permitted. The arrows indicate the upper limits on the sizes of the persistent source (left) and the star-forming region (right), respectively8,10. The parallel dashed lines represent fits to a range of Galactic and extragalactic H ii regions21. The parallel dotted lines represent the evolution of 1M⊙ and 10M⊙ of ejecta in up to 1,000 years at a velocity of 104 km s−1 in the blast-wave phase following a supernova53. The filled downward-pointing triangles and diamonds correspond to the supernova remnants Cas A and SN 1987A, respectively54,55. The filled circles represent the mean density and diameter of the Crab Nebula, whereas the filled squares represent the characteristic density and length scale of a dense filament in the Crab Nebula24. The stars indicate the density of Sagittarius A* at the Bondi radius22.
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Michilli, D., Seymour, A., Hessels, J. et al. An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102. Nature 553, 182–185 (2018). https://doi.org/10.1038/nature25149
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