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We present the results of the search for an isotropic stochasticgravitational wave background (GWB) at nanohertz frequencies using the seconddata release of the European Pulsar Timing Array (EPTA) for 25 millisecondpulsars and a combination with the first data release of the Indian PulsarTiming Array (InPTA). We analysed (i) the full 24.7-year EPTA data set, (ii)its 10.3-year subset based on modern observing systems, (iii) the combinationof the full data set with the first data release of the InPTA for ten commonlytimed millisecond pulsars, and (iv) the combination of the 10.3-year subsetwith the InPTA data. These combinations allowed us to probe the contributionsof instrumental noise and interstellar propagation effects. With the full dataset, we find marginal evidence for a GWB, with a Bayes factor of four and afalse alarm probability of $4\%$. With the 10.3-year subset, we report evidencefor a GWB, with a Bayes factor of $60$ and a false alarm probability of about$0.1\%$ ($\gtrsim 3\sigma$ significance). The addition of the InPTA data yieldsresults that are broadly consistent with the EPTA-only data sets, with thebenefit of better noise modelling. Analyses were performed with different dataprocessing pipelines to test the consistency of the results from independentsoftware packages. The inferred spectrum from the latest EPTA data from newgeneration observing systems is rather uncertain and in mild tension with thecommon signal measured in the full data set. However, if the spectral index isfixed at 13/3, the two data sets give a similar amplitude of($2.5\pm0.7)\times10^{-15}$ at a reference frequency of $1\,{\rm yr}^{-1}$. Bycontinuing our detection efforts as part of the International Pulsar TimingArray (IPTA), we expect to be able to improve the measurement of spatialcorrelations and better characterise this signal in the coming years.

The second data release from the European Pulsar Timing Array III. Search for gravitational wave signals