The second data release from the European Pulsar Timing Array

TLDR

The authors aim to detect an isotropic stochastic gravitational‑wave background by analysing the second European Pulsar Timing Array data release and combining it with the first Indian Pulsar Timing Array release as part of the International Pulsar Timing Array effort. They examined the full 24.7‑year EPTA data set, a 10.3‑year subset from modern systems, and each combined with InPTA data for ten common millisecond pulsars, employing multiple independent pipelines to assess instrumental and interstellar noise contributions. The full data set yields marginal GWB evidence (Bayes factor 4, 4 % false alarm), the 10.3‑year subset shows stronger evidence (Bayes factor 60, ~0.1 % false alarm, >3σ), and adding InPTA data produces consistent results with an amplitude of (2.5 ± 0.7) × 10⁻¹⁵ at 1 yr⁻¹ when the spectral index is fixed at 13/3.

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). 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 $0.1\%$ ($\gtrsim 3σ$ 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 inferred spectrum from the latest EPTA data from new generation observing systems 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 ($2.5\pm0.7)\times10^{-15}$ at a reference frequency of $1,{\rm yr}^{-1}$. 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.