The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background Zaven Arzoumanian, Paul T. Baker, Harsha Blumer, Bence Bécsy, Adam Brazier, Paul R. Brook, Sarah Burke-Spolaor, Shami Chatterjee, Siyuan Chen, James M. Cordes, et al. To cite this version: Zaven Arzoumanian, Paul T. Baker, Harsha Blumer, Bence Bécsy, Adam Brazier, et al.. The NANOGrav 12.5 yr Data Set: Search for an Isotropic Stochastic Gravitational-wave Background. Astrophys.J.Lett., 2020, 905 (2), pp.L34. 10.3847/2041-8213/abd401. hal-02953152 HAL Id: hal-02953152 https://hal.archives-ouvertes.fr/hal-02953152 Submitted on 30 Sep 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Draft version September 11, 2020 Typeset using LATEX twocolumn style in AASTeX63 The NANOGrav 12.5-year Data Set: Search For An Isotropic Stochastic Gravitational-Wave Background Zaven Arzoumanian,1 Paul T. Baker,2 Harsha Blumer,3, 4 Bence Becsy´ ,5 Adam Brazier,6 Paul R. Brook,3, 4 Sarah Burke-Spolaor,3, 4, 7 Shami Chatterjee,6 Siyuan Chen,8, 9, 10 James M. Cordes,6 Neil J. Cornish,5 Fronefield Crawford,11 H. Thankful Cromartie,12 Megan E. DeCesar,13, 14, ∗ Paul B. Demorest,15 Timothy Dolch,16 Justin A. Ellis,17 Elizabeth C. Ferrara,18 William Fiore,3, 4 Emmanuel Fonseca,19 Nathan Garver-Daniels,3, 4 Peter A. Gentile,3, 4 Deborah C. Good,20 Jeffrey S. Hazboun,21, ∗ A. Miguel Holgado,22 Kristina Islo,23 Ross J. Jennings,6 Megan L. Jones,23 Andrew R. Kaiser,3, 4 David L. Kaplan,23 Luke Zoltan Kelley,24 Joey Shapiro Key,21 Nima Laal,25 Michael T. Lam,26, 27 T. Joseph W. Lazio,28 Duncan R. Lorimer,3, 4 Jing Luo,29 Ryan S. Lynch,30 Dustin R. Madison,3, 4, ∗ Maura A. McLaughlin,3, 4 Chiara M. F. Mingarelli,31, 32 Cherry Ng,33 David J. Nice,13 Timothy T. Pennucci,34, 35, ∗ Nihan S. Pol,3, 4 Scott M. Ransom,34 Paul S. Ray,36 Brent J. Shapiro-Albert,3, 4 Xavier Siemens,25, 23 Joseph Simon,28, 37 Renee´ Spiewak,38 Ingrid H. Stairs,20 Daniel R. Stinebring,39 Kevin Stovall,15 Jerry P. Sun,25 Joseph K. Swiggum,13, ∗ Stephen R. Taylor,40 Jacob E. Turner,3, 4 Michele Vallisneri,28 Sarah J. Vigeland,23 Caitlin A. Witt,3, 4 The NANOGrav Collaboration 1X-Ray Astrophysics Laboratory, NASA Goddard Space Flight Center, Code 662, Greenbelt, MD 20771, USA 2Department of Physics and Astronomy, Widener University, One University Place, Chester, PA 19013, USA 3Department of Physics and Astronomy, West Virginia University, P.O. Box 6315, Morgantown, WV 26506, USA 4Center for Gravitational Waves and Cosmology, West Virginia University, Chestnut Ridge Research Building, Morgantown, WV 26505, USA 5Department of Physics, Montana State University, Bozeman, MT 59717, USA 6Cornell Center for Astrophysics and Planetary Science and Department of Astronomy, Cornell University, Ithaca, NY 14853, USA 7CIFAR Azrieli Global Scholars program, CIFAR, Toronto, Canada 8Station de Radioastronomie de Nancay, Observatoire de Paris, Universite PSL, CNRS, Universite d'Orleans, 18330 Nancay, France 9FEMTO-ST Institut de recherche, Department of Time and Frequency, UBFC and CNRS, ENSMM, 25030 Besancon, France 10Laboratoire de Physique et Chimie de l'Environment et de l'Espace, LPC2E UMR7328, Universite d'Orleans, CNRS, 45071 Orleans, France 11Department of Physics and Astronomy, Franklin & Marshall College, P.O. Box 3003, Lancaster, PA 17604, USA 12University of Virginia, Department of Astronomy, P.O. Box 400325, Charlottesville, VA 22904, USA 13Department of Physics, Lafayette College, Easton, PA 18042, USA 14AAAS, STPF, ORISE Fellow hosted by the U.S. Department of Energy 15National Radio Astronomy Observatory, 1003 Lopezville Rd., Socorro, NM 87801, USA 16Department of Physics, Hillsdale College, 33 E. College Street, Hillsdale, Michigan 49242, USA 17Infinia ML, 202 Rigsbee Avenue, Durham NC, 27701 18NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA 19Department of Physics, McGill University, 3600 University St., Montreal, QC H3A 2T8, Canada 20Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Road, Vancouver, BC V6T 1Z1, Canada 21University of Washington Bothell, 18115 Campus Way NE, Bothell, WA 98011, USA 22 arXiv:2009.04496v1 [astro-ph.HE] 9 Sep 2020 Department of Astronomy and National Center for Supercomputing Applications, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 23Center for Gravitation, Cosmology and Astrophysics, Department of Physics, University of Wisconsin-Milwaukee, P.O. Box 413, Milwaukee, WI 53201, USA 24Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University, Evanston, IL 60208 25Department of Physics, Oregon State University, Corvallis, OR 97331, USA 26School of Physics and Astronomy, Rochester Institute of Technology, Rochester, NY 14623, USA 27Laboratory for Multiwavelength Astronomy, Rochester Institute of Technology, Rochester, NY 14623, USA 28Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Corresponding author: Joseph Simon [email protected] 2 The NANOGrav Collaboration 29Department of Astronomy & Astrophysics, University of Toronto, 50 Saint George Street, Toronto, ON M5S 3H4, Canada 30Green Bank Observatory, P.O. Box 2, Green Bank, WV 24944, USA 31Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, New York, 10010, USA 32Department of Physics, University of Connecticut, 196 Auditorium Road, U-3046, Storrs, CT 06269-3046, USA 33Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George St., Toronto, ON M5S 3H4, Canada 34National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 35Institute of Physics, E¨otv¨osLor´andUniversity, P´azm´anyP. s. 1/A, 1117 Budapest, Hungary 36Space Science Division, Naval Research Laboratory, Washington, DC 20375-5352, USA 37Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO 80309, USA 38Centre for Astrophysics and Supercomputing, Swinburne University of Technology, P.O. Box 218, Hawthorn, Victoria 3122, Australia 39Department of Physics and Astronomy, Oberlin College, Oberlin, OH 44074, USA 40Department of Physics and Astronomy, Vanderbilt University, 2301 Vanderbilt Place, Nashville, TN 37235, USA ABSTRACT We search for an isotropic stochastic gravitational-wave background (GWB) in the 12:5-year pulsar timing data set collected by the North American Nanohertz Observatory for Gravitational Waves (NANOGrav). Our analysis finds strong evidence of a stochastic process, modeled as a power-law, with common amplitude and spectral slope across pulsars. The Bayesian posterior of the amplitude for a f −2=3 power-law spectrum, expressed as characteristic GW strain, has median 1:92 10−15 and −15 −1 × 5%{95% quantiles of 1:37{2:67 10 at a reference frequency of fyr = 1 yr . The Bayes factor in × favor of the common-spectrum process versus independent red-noise processes in each pulsar exceeds 10; 000. However, we find no statistically significant evidence that this process has quadrupolar spatial correlations, which we would consider necessary to claim a GWB detection consistent with General Relativity. We find that the process has neither monopolar nor dipolar correlations, which may arise from, for example, reference clock or solar-system ephemeris systematics, respectively. The amplitude posterior has significant support above previously reported upper limits; we explain this in terms of the Bayesian priors assumed for intrinsic pulsar red noise. We examine potential implications for the supermassive black hole binary population under the hypothesis that the signal is indeed astrophysical in nature. Keywords: Gravitational waves { Methods: data analysis { Pulsars: general 1. INTRODUCTION The North American Nanohertz Observatory for Pulsar-timing arrays (PTAs, Sazhin 1978; Detweiler Gravitational Waves (NANOGrav; Ransom et al. 2019) 1979; Foster & Backer 1990) seek to detect very-low- has been acquiring pulsar timing data since 2004. frequency ( 1{100 nHz) gravitational waves (GWs) by NANOGrav is one of three major PTAs along with the ∼ monitoring the spatially correlated fluctuations induced European Pulsar Timing Array (EPTA; Desvignes et al. by the waves on the times of arrival of radio pulses from 2016), and the Parkes Pulsar Timing Array (PPTA; millisecond pulsars (MSPs). The dominant source of Kerr et al. 2020). Additionally, there are growing PTA gravitational radiation in this band is expected to be the efforts in India (Joshi et al. 2018) and China (Lee stochastic background generated by a cosmic population 2016), as well as some telescope-centered timing pro- of supermassive black hole binaries (SMBHBs; Sesana grams (Bailes et al. 2018; Ng 2018). In concert, these et al. 2004; Burke-Spolaor et al. 2019). Other more collaborations support the International Pulsar Timing speculative stochastic GW sources in the nanohertz fre- Array (IPTA, Perera et al. 2019). Over the last decade, quency range include cosmic strings (Siemens et al. 2007; PTAs have produced