Miles, Matthew T.Shannon, Ryan M.Reardon, Daniel J.Bailes, MatthewChampion, David J.Geyer, MarisaGitika, PratyashaGrunthal, KathrinKeith, Michael J.Kramer, MichaelKulkarni, Atharva D.Nathan, Rowina S.Parthasarathy, AdityaSingha, JaikhombaTheureau, GillesThrane, EricAbbate, FedericoBuchner, SarahCameron, Andrew D.Camilo, FernandoMoreschi, Beatrice E.Shaifullah, GolamShamohammadi, MohsenPossenti, AndreaKrishnan, Vivek Venkatraman2026-02-192026-02-190035-8711WOS:001380378900001ORCID:/0000-0002-3922-2773/work/205904208https://hdl.handle.net/1885/733805691Pulsar timing arrays search for nanohertz-frequency gravitational waves by regularly observing ensembles of millisecond pulsars over many years to look for correlated timing residuals. Recently the first evidence for a stochastic gravitational wave background has been presented by the major arrays, with varying levels of significance (similar to 2 sigma-4 sigma). In this paper, we present the results of background searches with the MeerKAT Pulsar Timing Array. Although of limited duration (4.5 yr), the similar to 250000 arrival times with a median error of just 3 mu s on 83 pulsars make it very sensitive to spatial correlations. Detection of a gravitational wave background requires careful modelling of noise processes to ensure that any correlations represent a fit to the underlying background and not other misspecified processes. Under different assumptions about noise processes, we can produce either what appear to be compelling Hellings-Downs correlations of high significance (3 sigma-3.4 sigma) with a spectrum close to that which is predicted, or surprisingly, under slightly different assumptions, ones that are insignificant. This appears to be related to the fact that many of the highest precision MeerKAT Pulsar Timing Array pulsars are in close proximity and dominate the detection statistics. The sky-averaged characteristic strain amplitude of the correlated signal in our most significant model is h(c,yr)=7.5(-0.9)(+0.8)x10(-15) measured at a spectral index of alpha=-0.26, decreasing to h(c,yr)=4.8(-0.9)(+0.8)x10(-15) when assessed at the predicted alpha=-2/3. These data will be valuable as the International Pulsar Timing Array project explores the significance of gravitational wave detections and their dependence on the assumed noise models.The MeerKAT telescope is operated by the South African Radio Astronomy Observatory (SARAO), which is a facility of the National Research Foundation, an agency of the Department of Science and Innovation. SARAO acknowledges the ongoing advice and calibration of GPS systems by the National Metrology Institute of South Africa (NMISA) and the time-space reference systems department of the Paris Observatory. MeerTime data are stored and processed on the OzStar and Ngarrgu Tindebeek supercomputers, operated by the Swinburne University of Technology. This work was undertaken as part of the Australian Research Council (ARC) Centre of Excellence for Gravitational Wave Discovery (CE170100004 and CE230100016). RMS acknowledges support through ARC Future Fellowship FT190100155. GT acknowledges financial support from 'Programme National de Cosmologie and Galaxies' (PNCG), 'Programme National Hautes Energies' (PNHE), and 'Programme National Gravitation, References, Astronomie, Metrologie' funded by INSU,CNRS-IN2P3-INP, CEA, and CNES, France. We acknowledge financial support from Agence Nationale de la Recherche (ANR-18-CE31-0015), France. MK acknowledges support from the MPG and the CAS-MPG Legacy Programme. KG acknowledges continuing valuable support from the Max-Planck Society. KG also acknowledges the support from the International Max Planck Research School (IMPRS) for Astronomy and Astrophysics at the Universities of Bonn and Cologne. VVK acknowledges financial support from the European Research Council (ERC) starting grant 'COMPACT' (grant agreement number 101078094). FA acknowledges that part of the research activities described in this paper were carried out with the contribution of the NextGenerationEU funds within the National Recovery and Resilience Plan (PNRR), Mission 4 - Education and Research, Component 2 - From Research to Business (M4C2), Investment Line 3.1 - Strengthening and Creation of Research Infrastructures, Project IR0000034 - 'STILES - Strengthening the Italian Leadership in ELT and SKA'. Pulsar research at Jodrell Bank Centre for Astrophysics was supported by an STFC consolidated grants (ST/T000414/1 and ST/X001229/1). JS acknowledges funding from the South African Research Chairs Initiative of The Department of Science and Technology and the National Research Foundation of South Africa. PG acknowledges support through the Swinburne University of Technology (SUT) stipend, Swinburne University Postgraduate Research Award (SUPRA). AP acknowledges financial support from the European Research Council (ERC) starting grant 'GIGA' (grant agreement no. 101116134) and through the NWO-I Veni fellowship. Funding was provided for the PTUSE machines by . Funding was provided for the PTUSE machines by the Max-Planck-Institut fur Radioastronomie (MPlfR), also supported by the MPG-CAS LEGACY programme, Swinburne University of Technology, and the Australian SKA office. This project utilized the MeerTime data portal, which was supported by Nick Swainston and the ADACS team. We acknowledge and pay respects to the Elders and Traditional Owners of the land on which the Australian institutions stand, the Bunurong and Wurundjeri Peoples of the Kulin Nation. We thank Rutger van Haasteren, Steve Taylor, Siyuan Chen, and Levi Schult for their useful comments and discussions.12en©2024 The authorsGravitational wavesMethods: data analysisPulsars: generalStars: black holes2024-12-0310.1093/mnras/stae2571105012295685