title: Limits on Anisotropy in the Nanohertz Stochastic Gravitational Wave Background.
abstract: The paucity of observed supermassive black hole binaries (SMBHBs) may imply that the gravitational wave background (GWB) from this population is anisotropic, rendering existing analyses suboptimal. We present the first constraints on the angular distribution of a nanohertz stochastic GWB from circular, inspiral-driven SMBHBs using the 2015 European Pulsar Timing Array data. Our analysis of the GWB in the ~2-90 nHz band shows consistency with isotropy, with the strain amplitude in l>0 spherical harmonic multipoles ≲40% of the monopole value. We expect that these more general techniques will become standard tools to probe the angular distribution of source populations.
authors.affiliation: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA.|Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom.
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authors.initials: SR
authors.lastname: Taylor
authors.affiliation: TAPIR (Theoretical Astrophysics), California Institute of Technology, MC 350-17, Pasadena, California 91125, USA.|Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.|School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
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authors.lastname: Mingarelli
authors.affiliation: Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, United Kingdom.
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authors.initials: JR
authors.lastname: Gair
authors.affiliation: School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.|Max-Planck-Institut für Gravitationsphysik, Albert Einstein Institut, Am Mühlenberg 1, 14476 Golm, Germany.
authors.forename: A
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authors.affiliation: Laboratoire de Physique et Chimie de l'Environnement et de l'Espace LPC2E CNRS-Université d'Orléans, F-45071 Orléans, France.|Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, F-18330 Nançay, France.|Laboratoire Univers et Théories LUTh, Observatoire de Paris, CNRS/INSU, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France.
authors.forename: G
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authors.affiliation: Max-Planck-Institut für Gravitationsphysik, Albert Einstein Institut, Am Mühlenberg 1, 14476 Golm, Germany.
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authors.lastname: Babak
authors.affiliation: ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, Netherlands.|Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, United Kingdom.
authors.forename: C G
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authors.affiliation: Max-Planck-Institut für Gravitationsphysik, Albert Einstein Institut, Am Mühlenberg 1, 14476 Golm, Germany.
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authors.affiliation: INAF-Osservatorio Astronomico di Cagliari, via della Scienza 5, I-09047 Selargius, CA, Italy.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.affiliation: Laboratoire de Physique et Chimie de l'Environnement et de l'Espace LPC2E CNRS-Université d'Orléans, F-45071 Orléans, France.|Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, F-18330 Nançay, France.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.affiliation: Laboratoire de Physique et Chimie de l'Environnement et de l'Espace LPC2E CNRS-Université d'Orléans, F-45071 Orléans, France.|Station de radioastronomie de Nançay, Observatoire de Paris, CNRS/INSU, F-18330 Nançay, France.
authors.forename: L
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authors.lastname: Guillemot
authors.affiliation: ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, Netherlands.|Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands.
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authors.initials: JW
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authors.affiliation: ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, Netherlands.|Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, United Kingdom.
authors.forename: G H
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.|Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, United Kingdom.
authors.forename: M
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.|Laboratoire de Physique et Chimie de l'Environnement et de l'Espace LPC2E CNRS-Université d'Orléans, F-45071 Orléans, France.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.
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authors.lastname: Liu
authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.|Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany.
authors.forename: S
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authors.affiliation: INAF-Osservatorio Astronomico di Cagliari, via della Scienza 5, I-09047 Selargius, CA, Italy.
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authors.affiliation: Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, United Kingdom.
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authors.affiliation: Centre for Astrophysics and Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn VIC 3122, Australia.|Max Planck Institute for Gravitational Physics, Albert Einstein Institute, Callinstraße 38, 30167 Hanover, Germany.
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authors.affiliation: Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, United Kingdom.|Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, Netherlands.
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authors.affiliation: Jodrell Bank Centre for Astrophysics, University of Manchester, Manchester M13 9PL, United Kingdom.
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authors.affiliation: INAF-Osservatorio Astronomico di Cagliari, via della Scienza 5, I-09047 Selargius, CA, Italy.|Dipartimento di Fisica-Universitá di Cagliari, Cittadella Universitaria, I-09042 Monserrato, CA, Italy.
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authors.affiliation: School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, United Kingdom.
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authors.affiliation: Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany.|Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany.
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