Fast radio bursts1,2 are astronomical radio flashes of unknown physical nature with durations of milliseconds. Their dispersive arrival times suggest an extragalactic origin and imply radio luminosities that are orders of magnitude larger than those of all known short-duration radio transients3. So far all fast radio bursts have been detected with large single-dish telescopes with arcminute localizations, and attempts to identify their counterparts (source or host galaxy) have relied on the contemporaneous variability of field sources4 or the presence of peculiar field stars5 or galaxies4. These attempts have not resulted in an unambiguous association6,7 with a host or multi-wavelength counterpart. Here we report the subarcsecond localization of the fast radio burst FRB 121102, the only known repeating burst source8,9,10,11, using high-time-resolution radio interferometric observations that directly image the bursts. Our precise localization reveals that FRB 121102 originates within 100 milliarcseconds of a faint 180-microJansky persistent radio source with a continuum spectrum that is consistent with non-thermal emission, and a faint (twenty-fifth magnitude) optical counterpart. The flux density of the persistent radio source varies by around ten per cent on day timescales, and very long baseline radio interferometry yields an angular size of less than 1.7 milliarcseconds. Our observations are inconsistent with the fast radio burst having a Galactic origin or its source being located within a prominent star-forming galaxy. Instead, the source appears to be co-located with a low-luminosity active galactic nucleus or a previously unknown type of extragalactic source. Localization and identification of a host or counterpart has been essential to understanding the origins and physics of other kinds of transient events, including gamma-ray bursts12,13 and tidal disruption events14. However, if other fast radio bursts have similarly faint radio and optical counterparts, our findings imply that direct subarcsecond localizations may be the only way to provide reliable associations.

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The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities. We thank the staff at the NRAO for their continued support of these observations, especially with scheduling and computational infrastructure. 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. We thank the staff at Arecibo for their support and dedication that enabled these observations. Further acknowledgements of telescope facilities and funding agencies are included as Supplementary Information.

Author information


  1. Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, New York 14853, USA

    • S. Chatterjee
    • , R. S. Wharton
    •  & J. M. Cordes
  2. Department of Astronomy and Radio Astronomy Lab, University of California, Berkeley, California 94720, USA

    • C. J. Law
  3. National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA

    • S. Burke-Spolaor
    • , P. Demorest
    •  & B. J. Butler
  4. Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA

    • S. Burke-Spolaor
    •  & M. A. McLaughlin
  5. Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, West Virginia 26505, USA

    • S. Burke-Spolaor
    •  & M. A. McLaughlin
  6. ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands

    • J. W. T. Hessels
    •  & C. G. Bassa
  7. Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands

    • J. W. T. Hessels
  8. Academia Sinica Institute of Astronomy and Astrophysics, 645 North A’ohoku Place, Hilo, Hawaii 96720, USA

    • G. C. Bower
  9. Department of Physics and McGill Space Institute, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8, Canada

    • S. P. Tendulkar
    •  & V. M. Kaspi
  10. Arecibo Observatory, HC3 Box 53995, Arecibo, Puerto Rico 00612, USA

    • A. Seymour
  11. National Research Council of Canada, Herzberg Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, British Columbia V2A 6J9, Canada

    • P. Scholz
    •  & M. Rupen
  12. Haverford College, 370 Lancaster Avenue, Haverford, Pennsylvania 19041, USA

    • M. W. Abruzzo
  13. Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA

    • S. Bogdanov
  14. Joint Institute for VLBI ERIC, Postbus 2, 7990 AA Dwingeloo, The Netherlands

    • A. Keimpema
    • , B. Marcote
    • , Z. Paragi
    •  & H. J. van Langevelde
  15. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA

    • T. J. W. Lazio
  16. National Radio Astronomy Observatory, Charlottesville, Virginia 22903, USA

    • S. M. Ransom
  17. Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn D-53121, Germany

    • L. G. Spitler
  18. Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands

    • H. J. van Langevelde


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S.C. was principal investigator of the localization campaign described here. C.J.L. and S.B.-S. are principal investigators of the realfast project and performed the analysis that achieved the first VLA burst detections. S.C., C.J.L., R.S.W., S.B.-S., G.C.B., B.B. and P.D. performed detailed analysis of the VLA data. S.B.-S. and B.B. led the analysis of the VLA multi-band spectral data. J.W.T.H. was principal investigator of the EVN observations, which were analysed by Z.P. and B.M. G.C.B. was principal investigator of the VLBA observations, and led their analysis. J.W.T.H., A.S. and L.G.S. led the execution and analysis of the parallel Arecibo observing campaign. P.D. led the commissioning of fast-sampled VLA observing modes. S.C. was principal investigator of the ALMA observations. P.S. was principal investigator of the X-ray observations, and performed the X-ray analysis, along with S.B. S.P.T. was principal investigator of the Gemini observations, and along with C.G.B. led the analysis of Keck, Gemini and archival UKIDSS and GLIMPSE data. S.C. and C.J.L. led the writing of the manuscript, with substantial contributions from J.M.C. and J.W.T.H. All authors contributed substantially to the interpretation of the analysis results and to the final version of the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to S. Chatterjee.

Reviewer Information Nature thanks H. Falcke and G. Hallinan for their contribution to the peer review of this work.

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