The second data release from the European Pulsar Timing Array

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Volume 678 (October 2023)

A&A, 678 (2023) A50

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Issue		A&A Volume 678, October 2023


Article Number		A50
Number of page(s)		22
Section		Astrophysical processes
DOI		https://doi.org/10.1051/0004-6361/202346844
Published online		03 October 2023

A&A 678, A50 (2023)

III. Search for gravitational wave signals

EPTA Collaboration and InPTA Collaboration
J. Antoniadis (Ιωάννης Aντωνιάδης)¹^,2, P. Arumugam³, S. Arumugam⁴, S. Babak⁵, M. Bagchi⁶^,7, A.-S. Bak Nielsen²^,8, C. G. Bassa⁹, A. Bathula¹⁰, A. Berthereau¹¹^,12, M. Bonetti¹³^,14^,15, E. Bortolas¹³^,14^,15, P. R. Brook¹⁶, M. Burgay¹⁷, R. N. Caballero¹⁸, A. Chalumeau¹³, D. J. Champion², S. Chanlaridis¹, S. Chen¹⁹^,⋆, I. Cognard¹¹^,12, S. Dandapat²⁰, D. Deb⁶, S. Desai²¹, G. Desvignes², N. Dhanda-Batra²², C. Dwivedi²³, M. Falxa⁵^,11, R. D. Ferdman²⁴, A. Franchini¹³^,14, J. R. Gair²⁵, B. Goncharov²⁶^,27, A. Gopakumar²⁰, E. Graikou², J.-M. Grießmeier¹¹^,12, L. Guillemot¹¹^,12, Y. J. Guo²^,⋆, Y. Gupta²⁸, S. Hisano²⁹, H. Hu², F. Iraci³⁰^,17, D. Izquierdo-Villalba¹³^,14, J. Jang², J. Jawor², G. H. Janssen⁹^,31, A. Jessner², B. C. Joshi²⁸^,3, F. Kareem³²^,33, R. Karuppusamy², E. F. Keane³⁴, M. J. Keith³⁵, D. Kharbanda²¹, T. Kikunaga²⁹, N. Kolhe³⁶, M. Kramer²^,35, M. A. Krishnakumar²^,8, K. Lackeos², K. J. Lee³⁷^,38, K. Liu², Y. Liu³⁸^,8, A. G. Lyne³⁵, J. W. McKee³⁹^,40, Y. Maan²⁸, R. A. Main², M. B. Mickaliger³⁵, I. C. Niţu³⁵, K. Nobleson⁴¹, A. K. Paladi⁴², A. Parthasarathy², B. B. P. Perera⁴³, D. Perrodin¹⁷, A. Petiteau⁴⁴^,5, N. K. Porayko¹³^,2, A. Possenti¹⁷, T. Prabu⁴⁵, H. Quelquejay Leclere⁵, P. Rana²⁰, A. Samajdar⁴⁶, S. A. Sanidas³⁵, A. Sesana¹³^,14^,15, G. Shaifullah¹³^,14^,17, J. Singha³, L. Speri²⁵, R. Spiewak³⁵, A. Srivastava²¹, B. W. Stappers³⁵, M. Surnis⁴⁷, S. C. Susarla⁴⁸, A. Susobhanan⁴⁹, K. Takahashi⁵⁰^,51, P. Tarafdar⁶, G. Theureau¹¹^,12^,52, C. Tiburzi¹⁷, E. van der Wateren⁹^,31, A. Vecchio¹⁶, V. Venkatraman Krishnan², J. P. W. Verbiest⁵³^,8^,2, J. Wang⁸^,54^,55, L. Wang³⁵ and Z. Wu³⁸^,8

¹ Institute of Astrophysics, FORTH, N. Plastira 100, 70013 Heraklion, Greece
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Department of Physics, Indian Institute of Technology Roorkee, Roorkee 247667, India
⁴ Department of Electrical Engineering, IIT Hyderabad, Kandi, Telangana 502284, India
⁵ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁶ The Institute of Mathematical Sciences, C. I. T. Campus, Taramani, Chennai 600113, India
⁷ Homi Bhabha National Institute, Training School Complex, Anushakti Nagar, Mumbai 400094, India
⁸ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁹ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
¹⁰ Department of Physical Sciences, Indian Institute of Science Education and Research, Mohali, Punjab 140306, India
¹¹ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace, Université d’Orléans/CNRS, 45071 Orléans Cedex 02, France
¹² Observatoire Radioastronomique de Nançay, Observatoire de Paris, Université PSL, Université d’Orléans, CNRS, 18330 Nançay, France
¹³ Dipartimento di Fisica “G. Occhialini”, Universitá degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁴ INFN, Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
¹⁵ INAF – Osservatorio Astronomico di Brera, via Brera 20, 20121 Milano, Italy
¹⁶ Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
¹⁷ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius (CA), Italy
¹⁸ Hellenic Open University, School of Science and Technology, 26335 Patras, Greece
¹⁹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
²⁰ Department of Astronomy and Astrophysics, Tata Institute of Fundamental Research, Homi Bhabha Road, Navy Nagar, Colaba, Mumbai 400005, India
²¹ Department of Physics, IIT Hyderabad, Kandi, Telangana 502284, India
²² Department of Physics and Astrophysics, University of Delhi, Delhi 110007, India
²³ Department of Earth and Space Sciences, Indian Institute of Space Science and Technology, Valiamala, Thiruvananthapuram, Kerala 695547, India
²⁴ School of Physics, Faculty of Science, University of East Anglia, Norwich NR4 7TJ, UK
²⁵ Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Am Mühlenberg 1, 14476 Potsdam, Germany
²⁶ Gran Sasso Science Institute (GSSI), 67100 L’Aquila, Italy
²⁷ INFN, Laboratori Nazionali del Gran Sasso, 67100 Assergi, Italy
²⁸ National Centre for Radio Astrophysics, Pune University Campus, Pune 411007, India
²⁹ Kumamoto University, Graduate School of Science and Technology, Kumamoto 860-8555, Japan
³⁰ Universitá di Cagliari, Dipartimento di Fisica, S.P. Monserrato-Sestu Km 0,700, 09042 Monserrato (CA), Italy
³¹ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³² Department of Physical Sciences,Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
³³ Center of Excellence in Space Sciences India, Indian Institute of Science Education and Research Kolkata, Kolkata 741246, India
³⁴ School of Physics, Trinity College Dublin, College Green, Dublin 2 D02 PN40, Ireland
³⁵ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK
³⁶ Department of Physics, St. Xavier’s College (Autonomous), Mumbai 400001, India
³⁷ Department of Astronomy, School of Physics, Peking University, Beijing 100871, PR China
³⁸ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, PR China
³⁹ E.A. Milne Centre for Astrophysics, University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK
⁴⁰ Centre of Excellence for Data Science, Artificial Intelligence and Modelling (DAIM), University of Hull, Cottingham Road, Kingston-upon-Hull HU6 7RX, UK
⁴¹ Department of Physics, BITS Pilani Hyderabad Campus, Hyderabad 500078 Telangana, India
⁴² Joint Astronomy Programme, Indian Institute of Science, Bengaluru, Karnataka 560012, India
⁴³ Arecibo Observatory, HC3 Box 53995, Arecibo, PR 00612, USA
⁴⁴ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
⁴⁵ Raman Research Institute India, Bengaluru, Karnataka 560080, India
⁴⁶ Institut für Physik und Astronomie, Universität Potsdam, Haus 28, Karl-Liebknecht-Str. 24/25, 14476 Potsdam, Germany
⁴⁷ Department of Physics, IISER Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066 Madhya Pradesh, India
⁴⁸ Ollscoil na Gaillimhe – University of Galway, University Road, Galway H91 TK33, Ireland
⁴⁹ Center for Gravitation, Cosmology, and Astrophysics, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA
⁵⁰ Division of Natural Science, Faculty of Advanced Science and Technology, Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan
⁵¹ International Research Organization for Advanced Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
⁵² Laboratoire Univers et Théories LUTh, Observatoire de Paris, Université PSL, CNRS, Université de Paris, 92190 Meudon, France
⁵³ Florida Space Institute, University of Central Florida, 12354 Research Parkway, Partnership 1 Building, Suite 214, Orlando 32826-0650 FL, USA
⁵⁴ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute (AIRUB), 44780 Bochum, Germany
⁵⁵ Advanced Institute of Natural Sciences, Beijing Normal University, Zhuhai 519087, PR China

Received: 8 May 2023
Accepted: 23 June 2023

Abstract

We present the results of the search for an isotropic stochastic gravitational wave background (GWB) at nanohertz frequencies using the second data release of the European Pulsar Timing Array (EPTA) for 25 millisecond pulsars and a combination with the first data release of the Indian Pulsar Timing Array (InPTA). A robust GWB detection is conditioned upon resolving the Hellings-Downs angular pattern in the pairwise cross-correlation of the pulsar timing residuals. Additionally, the GWB is expected to yield the same (common) spectrum of temporal correlations across pulsars, which is used as a null hypothesis in the GWB search. Such a common-spectrum process has already been observed in pulsar timing data. We analysed (i) the full 24.7-year EPTA data set, (ii) its 10.3-year subset based on modern observing systems, (iii) the combination of the full data set with the first data release of the InPTA for ten commonly timed millisecond pulsars, and (iv) the combination of the 10.3-year subset with the InPTA data. These combinations allowed us to probe the contributions of instrumental noise and interstellar propagation effects. With the full data set, we find marginal evidence for a GWB, with a Bayes factor of four and a false alarm probability of 4%. With the 10.3-year subset, we report evidence for a GWB, with a Bayes factor of 60 and a false alarm probability of about 0.1% (≳3σ significance). The addition of the InPTA data yields results that are broadly consistent with the EPTA-only data sets, with the benefit of better noise modelling. Analyses were performed with different data processing pipelines to test the consistency of the results from independent software packages. The latest EPTA data from new generation observing systems show non-negligible evidence for the GWB. At the same time, the inferred spectrum is rather uncertain and in mild tension with the common signal measured in the full data set. However, if the spectral index is fixed at 13/3, the two data sets give a similar amplitude of (2.5 ± 0.7) × 10⁻¹⁵ at a reference frequency of 1 yr⁻¹. Further investigation of these issues is required for reliable astrophysical interpretations of this signal. By continuing our detection efforts as part of the International Pulsar Timing Array (IPTA), we expect to be able to improve the measurement of spatial correlations and better characterise this signal in the coming years.

Key words: gravitational waves / methods: data analysis / pulsars: general

Note to the reader: Affiliations list was mistakenly incorrect in the HTML version, although it was correct in the PDF file. Affiliations were corrected on March 18, 2024.

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Corresponding authors: S. Chen, e-mail: sychen@pku.edu.cn and Y. J. Guo, e-mail: yjguo@mpifr-bonn.mpg.de

Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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