title: A direct localization of a fast radio burst and its host.
abstract: Fast radio bursts 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 transients. 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 sources or the presence of peculiar field stars or galaxies. These attempts have not resulted in an unambiguous association 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 source, 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 bursts and tidal disruption events. 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.
authors.affiliation: Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, New York 14853, USA.
authors.forename: S
authors.initials: S
authors.lastname: Chatterjee
authors.affiliation: Department of Astronomy and Radio Astronomy Lab, University of California, Berkeley, California 94720, USA.
authors.forename: C J
authors.initials: CJ
authors.lastname: Law
authors.affiliation: Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, New York 14853, USA.
authors.forename: R S
authors.initials: RS
authors.lastname: Wharton
authors.affiliation: National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA.|Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA.|Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, West Virginia 26505, USA.
authors.forename: S
authors.initials: S
authors.lastname: Burke-Spolaor
authors.affiliation: ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands.|Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands.
authors.forename: J W T
authors.initials: JW
authors.lastname: Hessels
authors.affiliation: Academia Sinica Institute of Astronomy and Astrophysics, 645 North A'ohoku Place, Hilo, Hawaii 96720, USA.
authors.forename: G C
authors.initials: GC
authors.lastname: Bower
authors.affiliation: Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, New York 14853, USA.
authors.forename: J M
authors.initials: JM
authors.lastname: Cordes
authors.affiliation: Department of Physics and McGill Space Institute, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8, Canada.
authors.forename: S P
authors.initials: SP
authors.lastname: Tendulkar
authors.affiliation: ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands.
authors.forename: C G
authors.initials: CG
authors.lastname: Bassa
authors.affiliation: National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA.
authors.forename: P
authors.initials: P
authors.lastname: Demorest
authors.affiliation: National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA.
authors.forename: B J
authors.initials: BJ
authors.lastname: Butler
authors.affiliation: Arecibo Observatory, HC3 Box 53995, Arecibo, Puerto Rico 00612, USA.
authors.forename: A
authors.initials: A
authors.lastname: Seymour
authors.affiliation: National Research Council of Canada, Herzberg Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, British Columbia V2A 6J9, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Scholz
authors.affiliation: Haverford College, 370 Lancaster Avenue, Haverford, Pennsylvania 19041, USA.
authors.forename: M W
authors.initials: MW
authors.lastname: Abruzzo
authors.affiliation: Columbia Astrophysics Laboratory, Columbia University, New York, New York 10027, USA.
authors.forename: S
authors.initials: S
authors.lastname: Bogdanov
authors.affiliation: Department of Physics and McGill Space Institute, McGill University, 3600 University Street, Montreal, Quebec H3A 2T8, Canada.
authors.forename: V M
authors.initials: VM
authors.lastname: Kaspi
authors.affiliation: Joint Institute for VLBI ERIC, Postbus 2, 7990 AA Dwingeloo, The Netherlands.
authors.forename: A
authors.initials: A
authors.lastname: Keimpema
authors.affiliation: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.
authors.forename: T J W
authors.initials: TJ
authors.lastname: Lazio
authors.affiliation: Joint Institute for VLBI ERIC, Postbus 2, 7990 AA Dwingeloo, The Netherlands.
authors.forename: B
authors.initials: B
authors.lastname: Marcote
authors.affiliation: Department of Physics and Astronomy, West Virginia University, Morgantown, West Virginia 26506, USA.|Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, West Virginia 26505, USA.
authors.forename: M A
authors.initials: MA
authors.lastname: McLaughlin
authors.affiliation: Joint Institute for VLBI ERIC, Postbus 2, 7990 AA Dwingeloo, The Netherlands.
authors.forename: Z
authors.initials: Z
authors.lastname: Paragi
authors.affiliation: National Radio Astronomy Observatory, Charlottesville, Virginia 22903, USA.
authors.forename: S M
authors.initials: SM
authors.lastname: Ransom
authors.affiliation: National Research Council of Canada, Herzberg Astronomy and Astrophysics, Dominion Radio Astrophysical Observatory, PO Box 248, Penticton, British Columbia V2A 6J9, Canada.
authors.forename: M
authors.initials: M
authors.lastname: Rupen
authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, Bonn D-53121, Germany.
authors.forename: L G
authors.initials: LG
authors.lastname: Spitler
authors.affiliation: Joint Institute for VLBI ERIC, Postbus 2, 7990 AA Dwingeloo, The Netherlands.|Sterrewacht Leiden, Leiden University, Postbus 9513, 2300 RA Leiden, The Netherlands.
authors.forename: H J
authors.initials: HJ
authors.lastname: van Langevelde