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The Parkes Pulsar Timing Array (PPTA) is a major project on the Parkes Radio Telescope (Murriyang), with the primary aim of detecting nanohertz-frequency gravitational waves (GWs). A background of GWs (GWB) modulates pulsar arrival times and manifests as a stochastic process, common to all pulsars, with a signature spatial correlation. In this talk I describe our recent search for an isotropic stochastic GWB using the third data release of the PPTA. We recover a noise process with a common spectrum among the pulsars, which exhibits characteristics consistent with that expected of a GWB from supermassive black hole binaries. We also search for the spatial correlations expected from a GWB and estimate the significance of these correlations as 2 sigma in our data set. However, contrary to expectations for the GWB from an isotropic population of inspiralling supermassive black hole binaries, we demonstrate that the apparent signal strength is changing with time. I will discuss these results in the context of recent analyses from other international pulsar timing array collaborations and describe the exciting outlook for nanohertz gravitational wave astronomy with the sensitive new MeerKAT pulsar timing array. |
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