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

TL;DR

This paper reports a search for continuous gravitational waves in the EPTA DR2 data, identifying a candidate signal around 4-5 nHz, but with inconclusive evidence due to potential confusion with gravitational wave background signals.

Contribution

The study introduces a combined Bayesian and frequentist analysis approach to search for CGWs in PTA data and discusses the challenges of distinguishing signals from background noise.

Findings

Identified a candidate gravitational wave signal at 4-5 nHz.

Bayesian analysis shows slight favor for a CGW presence, but not conclusively.

Simulations reveal potential misinterpretation between CGW signals and gravitational wave background.

Abstract

We present the results of a search for continuous gravitational wave signals (CGWs) in the second data release (DR2) of the European Pulsar Timing Array (EPTA) collaboration. The most significant candidate event from this search has a gravitational wave frequency of 4-5 nHz. Such a signal could be generated by a supermassive black hole binary (SMBHB) in the local Universe. We present the results of a follow-up analysis of this candidate using both Bayesian and frequentist methods. The Bayesian analysis gives a Bayes factor of 4 in favor of the presence of the CGW over a common uncorrelated noise process, while the frequentist analysis estimates the p-value of the candidate to be 1%, also assuming the presence of common uncorrelated red noise. However, comparing a model that includes both a CGW and a gravitational wave background (GWB) to a GWB only, the Bayes factor in favour of the CGW model is only 0.7. Therefore, we cannot conclusively determine the origin of the observed feature, but we cannot rule it out as a CGW source. We present results of simulations that demonstrate that data containing a weak gravitational wave background can be misinterpreted as data including a CGW and vice versa, providing two plausible explanations of the EPTA DR2 data. Further investigations combining data from all PTA collaborations will be needed to reveal the true origin of this feature.