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Published March 2022 | Accepted Version + Published
Journal Article Open

The International Pulsar Timing Array second data release: Search for an isotropic gravitational wave background

Antoniadis, J. ORCID icon
Arzoumanian, Z.
Babak, S.
Bailes, M.
Bak Nielsen, A.-S. ORCID icon
Baker, P. T.
Bassa, C. G.
Bécsy, B.
Berthereau, A.
Bonetti, M. ORCID icon
Brazier, A.
Brook, P. R. ORCID icon
Burgay, M. ORCID icon
Burke-Spolaor, S.
Caballero, R. N. ORCID icon
Casey-Clyde, J. A.
Chalumeau, A. ORCID icon
Champion, D. J. ORCID icon
Charisi, M.
Chatterjee, S.
Chen, S. ORCID icon
Cognard, I.
Cordes, J. M.
Cornish, N. J.
Crawford, F. ORCID icon
Cromartie, H. T.
Crowter, K. ORCID icon
Dai, S. ORCID icon
DeCesar, M. E.
Demorest, P. B.
Desvignes, G. ORCID icon
Dolch, T.
Drachler, B.
Falxa, M.
Ferrara, E. C.
Fiore, W.
Fonseca, E.
Gair, J. R.
Garver-Daniels, N.
Goncharov, B. ORCID icon
Good, D. C.
Graikou, E.
Guillemot, L.
Guo, Y. J.
Hazboun, J. S. ORCID icon
Hobbs, G. ORCID icon
Hu, H. ORCID icon
Islo, K.
Janssen, G. H.
Jennings, R. J.
Johnson, A. D.
Jones, M. L.
Kaiser, A. R.
Kaplan, D. L. ORCID icon
Karuppusamy, R.
Keith, M. J.
Kelley, L. Z. ORCID icon
Kerr, M. ORCID icon
Key, J. S.
Kramer, M. ORCID icon
Lam, M. T.
Lamb, W. G.
Lazio, T. J. W. ORCID icon
Lee, K. J.
Lentati, L.
Liu, K.
Luo, J.
Lynch, R. S.
Lyne, A. G.
Madison, D. R.
Main, R. A.
Manchester, R. N.
McEwen, A.
McKee, J. W. ORCID icon
McLaughlin, M. A. ORCID icon
Mickaliger, M. B.
Mingarelli, C. M. F. ORCID icon
Ng, C. ORCID icon
Nice, D. J.
Osłowski, S.
Parthasarathy, A. ORCID icon
Pennucci, T. T.
Perera, B. B. P. ORCID icon
Perrodin, D.
Petiteau, A.
Pol, N. S.
Porayko, N. K.
Possenti, A.
Ransom, S. M. ORCID icon
Ray, P. S. ORCID icon
Reardon, D. J. ORCID icon
Russell, C. J.
Samajdar, A.
Sampson, L. M.
Sanidas, S.
Sarkissian, J. M.
Schmitz, K.
Schult, L. ORCID icon
Sesana, A.
Shaifullah, G. ORCID icon
Shannon, R. M. ORCID icon
Shapiro-Albert, B. J.
Siemens, X.
Simon, J. ORCID icon
Smith, T. L.
Speri, L. ORCID icon
Spiewak, R. ORCID icon
Stairs, I. H.
Stappers, B. W. ORCID icon
Stinebring, D. R.
Swiggum, J. K.
Taylor, S. R.
Theureau, G.
Tiburzi, C.
Vallisneri, M. ORCID icon
van der Wateren, E.
Vecchio, A.
Verbiest, J. P. W. ORCID icon
Vigeland, S. J.
Wahl, H.
Wang, J. B.
Wang, J.
Wang, L. ORCID icon
Witt, C. A.
Zhang, S.
Zhu, X. J. ORCID icon

Abstract

We searched for an isotropic stochastic gravitational wave background in the second data release of the International Pulsar Timing Array, a global collaboration synthesizing decadal-length pulsar-timing campaigns in North America, Europe, and Australia. In our reference search for a power law strain spectrum of the form h_c = A(f/1yr⁻¹)^α, we found strong evidence for a spectrally-similar low-frequency stochastic process of amplitude A =3.8^(+6.3)_(−2.5) × 10⁻¹⁵ and spectral index α =− 0.5 ± 0.5, where the uncertainties represent 95% credible regions, using information from the auto- and cross-correlation terms between the pulsars in the array. For a spectral index of α = −2/3, as expected from a population of inspiralling supermassive black hole binaries, the recovered amplitude is A = 2.8^(+1.2)_(−0.8) × 1010⁻¹⁵. Nonetheless, no significant evidence of the Hellings-Downs correlations that would indicate a gravitational-wave origin was found. We also analyzed the constituent data from the individual pulsar timing arrays in a consistent way, and clearly demonstrate that the combined international data set is more sensitive. Furthermore, we demonstrate that this combined data set produces comparable constraints to recent single-array data sets which have more data than the constituent parts of the combination. Future international data releases will deliver increased sensitivity to gravitational wave radiation, and significantly increase the detection probability.

Additional Information

© 2022 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) Accepted 2021 No v ember 15. Received 2021 November 4; in original form 2021 September 23. The International Pulsar Timing Array (IPTA) is a consortium of existing Pulsar Timing Array collaborations, namely, the European Pulsar Timing Array (EPTA), North American Nanohertz Observatory for Gravitational Waves (NANOGrav), Parkes Pulsar Timing Array (PPTA), and the recent addition of the Indian Pulsar Timing Array (InPTA). Observing collaborations from China and South Africa are also part of the IPTA. The EPTA is a collaboration between European and partner institutes with the aim to provide high precision pulsar timing to work towards the direct detection of low-frequency gravitational waves. An Advanced Grant of the European Research Council to implement the Large European Array for Pulsars (LEAP) also provides funding. Part of this work is based on observations with the 100-m telescope of the Max-Planck-Institut für Radioastronomie (MPIfR) at Effelsberg in Germany. Pulsar research at the Jodrell Bank Centre for Astrophysics and the observations using the Lovell Telescope are supported by a Consolidated Grant (ST/T000414/1) from the UK's Science and Technology Facilities Council. The Nançay radio Observatory is operated by the Paris Observatory, associated to the French Centre National de la Recherche Scientifique (CNRS), and partially supported by the Region Centre in France. We acknowledge financial support from 'Programme National de Cosmologie and Galaxies' (PNCG), and 'Programme National Hautes Energies' (PNHE) funded by CNRS/INSU-IN2P3-INP, CEA and CNES, France. We acknowledge financial support from Agence Nationale de la Recherche (ANR-18-CE31-0015), France. The Westerbork Synthesis Radio Telescope is operated by the Netherlands Institute for Radio Astronomy (ASTRON) with support from the Netherlands Foundation for Scientific Research (NWO). The Sardinia Radio Telescope (SRT) is funded by the Department of University and Research (MIUR), the Italian Space Agency (ASI), and the Autonomous Region of Sardinia (RAS) and is operated as National Facility by the National Institute for Astrophysics (INAF). The NANOGrav Physics Frontiers Center is supported by NSF award number 1430284. The Green Bank Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. The Arecibo Observatory is a facility of the National Science Foundation operated under cooperative agreement by the University of Central Florida in alliance with Yang Enterprises, Inc. and Universidad Metropolitana. The Parkes radio telescope (Murriyang) is part of the Australia Telescope which is funded by the Commonwealth Government for operation as a National Facility managed by CSIRO. JA acknowleges support by the Stavros Niarchos Foundation (SNF) and the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the 2nd Call of 'Science and Society' Action Always strive for excellence – 'Theodoros Papazoglou' (Project Number: 01431). SBS acknowledges generous support by the NSF through grant AST-1815664. The work is supported by National SKA program of China 2020SKA0120100, Max-Planck Partner Group, NSFC 11690024, CAS Cultivation Project for FAST Scientific. JACC was supported in part by NASA CT Space Grant PTE Federal Award Number 80NSSC20M0129. CMFM and JACC are also supported by the National Science Foundation's NANOGrav Physics Frontier Center, Award Number 2020265. AC acknowledges support from the Paris Île-de-France Region. Support for HTC was provided by NASA through the NASA Hubble Fellowship Program grant HST-HF2-51453.001 awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS5-26555. SD is the recipient of an Australian Research Council Discovery Early Career Award (DE210101738) funded by the Australian Government. GD, RK and MKr acknowledge support from European Research Council (ERC) Synergy Grant 'BlackHoleCam' Grant Agreement Number 610058 and ERC Advanced Grant 'LEAP' Grant Agreement Number 337062. TD is supported by the NSF AAG award number 2009468. ECF is supported by NASA under award number 80GSFC17M0002.002. BG is supported by the Italian Ministry of Education, University and Research within the PRIN 2017 Research Program Framework, n. 2017SYRTCN. Portions of this work performed at the Naval Research Laboratory is supported by NASA and ONR 6.1 basic research funding. MTL acknowledges support received from NSF AAG award number 200968. 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. JWMK is a CITA Postdoctoral Fellow: This work was supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), (funding reference CITA 490888-16). ASa, ASe, and GS acknowledge financial support provided under the European Union's H2020 ERC Consolidator Grant 'Binary Massive Black Hole Astrophysic' (B Massive, Grant Agreement: 818691). RMS acknowledges support through Australian Research Council Future Fellowship FT190100155. JS acknowledges support from the JPL R&TD program. This research was funded partially by the Australian Government through the Australian Research Council (ARC), grants CE170100004 (OzGrav) and FL150100148. Pulsar research at UBC is supported by an NSERC Discovery Grant and by the Canadian Institute for Advanced Research. SRT acknowledges support from NSF grants AST-2007993 and PHY-2020265. SRT also acknowledges support from the Vanderbilt University College of Arts & Science Dean's Faculty Fellowship program. AV acknowledges the support of the Royal Society and Wolfson Foundation. JPWV acknowledges support by the Deutsche Forschungsgemeinschaft (DFG) through the Heisenberg programme (Project No. 433075039). Author Contributions: An alphabetical-order author list was used for this paper to recognize the large amount of work and efforts contributed by many people within the IPTA consortium. All authors have contributed to the work via data creation, combination or analysis or contributions to the paper. The data set was created by a team led by BBPP, MED, PBD, MKe, LL, DJN, SO, SMR, and MJK, by combining data sets from the constituent PTAs (EPTA, NANOGrav, and PPTA). The data analysis has been the task of the Gravitational Wave Analysis Working Group (WG) of the IPTA. Many analyses have been performed over more than 2 yr by PTB, AC, SChe, JAE, JMG, JSH, KI, ADJ, WGL, CMFM, NSP, NKP, DJR, LSc, JS, LSp, SRT, and SJV under the leadership of several WG chairs over this time period: PTB, SChe, JSH, PDL, CMFM, DJR, and SRT. Initial tests, checks of the data quality and exploratory analyses were made by PTB, SChe, JAE, JMG, JSH, KI, CMFM, DJR, JS, SRT, and SJV. Although not much of the work is presented in this paper, it helped us tremendously in understanding the subtle details of the data set and built confidence in our analysis. The final results shown come from runs performed by AC, SChe, JSH, ADJ, WGL, NSP, NKP, ASa, LSc, GS, LSp, and SRT. The single pulsar noise analysis was done by SChe. The Bayesian parameter estimation were done by NSP, JSH, and WGL. The optimal statistic analysis was run by JSH and NSP, with help from AC, NKP, and LSp for the phase-shifts and sky-scramble null-distributions. ADJ and JSH have contributed to the Bayes factor computations. The factorized likelihood method has been applied to the data by LSc and SRT. NSP has computed the dropout factors. SChe performed the analyses of the constituent data sets, with cross-checks from ASa and GS. PTB lead the comparison between the IPTA DR2 and recent PTA data sets, which were graciously provided by JS for the NANOGrav 12.5 yr, BG for the PPTA DR2 and SChe for the EPTA DR2. The astrophysical interpretation was lead by JACC and CMFM with contributions from SChe and ASe. SChe lead the coordination of the paper writing with the help of JSH and PTB. PTB, JACC, SChe, MED, BG, JSH, CMFM, BBPP, NSP, LSc, RMS, SRT, and SJV contributed to the writing of the paper. The figures were created by NSP, JSH, JACC, and SChe. DATA AVAILABILITY. The timing data used can be found on https://gitlab.com/IPTA/DR2. Selected chain files for the analyses presented in this paper can be found on https://ipta4gw.org and 10.5281/zenodo.5787557.

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Additional details

Created:
August 22, 2023
Modified:
October 23, 2023