Evidence for different gravitational-wave sources in the NANOGrav dataset

The NANOGrav Collaboration recently reported a strong evidence for a stochastic common-spectrum process in the pulsar-timing data. We evaluate the evidence of interpreting this process as mergers of super massive black hole binaries and/or various stochastic gravitational-wave background sources in the early Universe, including first-order phase transitions, cosmic strings, domain walls, and large amplitude curvature perturbations. We discuss the implications of the constraints on these possible sources. It is found that the data shows a positive evidence in favor of the cosmic strings against other gravitational-wave sources based on the Bayes factor analysis.