2022ApJ...936...20C

We perform a direct search for an isotropic stochastic gravitational-wave background (SGWB) produced by cosmic strings in the Parkes Pulsar Timing Array (PPTA) Data Release 2 (DR2). We find no evidence for such an SGWB, and therefore place a 95% confidence level upper limit on the cosmic string tension, Gμ, as a function of the reconnection probability, p, which can be less than 1 in the string-theory-inspired models or pure Yang-Mills theory. The upper bound on the cosmic string tension is Gμ ≲ 5.1 × 10^–10 for p = 1, which is about five orders of magnitude tighter than the bound derived from the null search of individual gravitational-wave bursts from cosmic string cusps in the PPTA DR2, and comparable to previous bounds derived from the null search of the SGWB from cosmic strings.