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.
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.