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Composite topological structures such as superheavy "quasi-stable strings"(QSS) and "walls bounded by strings" (WBS) arise in realistic extensions of theStandard Model of high energy physics. We show that the gravitational radiationemitted in the early universe by these two unstable structures with adimensionless string tension $G\mu\approx 10^{-6}$ is consistent with theNANOGrav evidence of low frequency gravitational background as well as therecent LIGO-VIRGO constraints, provided the superheavy strings and monopolesexperience a certain amount of inflation. For the case of walls bounded bystrings, the domain walls arise from the spontaneous breaking of a remnantdiscrete gauge symmetry around the electroweak scale. The quasi-stable strings,on the other hand, arise from a two step breaking of a local gauge symmetry.The monopoles appear from the first breaking and get connected to strings thatarise from the second breaking. Both composite structures decay by emittinggravitational waves over a wide frequency range. The Bayes factors for QSS andWBS relative to the inspiraling supermassive black hole binaries are estimatedto be about 60 and 30 respectively, which are comparable with that ofmetastable strings and cosmic superstrings.

Superheavy quasi-stable strings and walls bounded by strings in the light of NANOGrav 15 year data