FIG. 4: Ω$_{gw}$ in the case where strings emerge at the end of inflation for Gµ = 10 − $^{7}$, T$_{r}$ ∼ 10 MeV. The upper line represents the estimate including “rare bursts”, and the lower line represents the estimate excluding “rare bursts”. GW background is much larger than that of kinks if they coexist in some frequency band. However, a loop cannot emit GWs with frequencies smaller than the inverse of its size. Thus there is a cut on the low frequency side of the spectrum of GWs from loops corresponding to the inverse of the loop size ∼ ( αt ) − $^{1}$. If α ∼ Gµ , the spectrum of GWs from loops begins to appear at ω ≳ 10 − 12 Hz, and this covers the frequency band where both pulsar timing arrays and GW detectors have good sensitivity. However, α is one of the most unknown parameters in the cosmic string model. According to some recent simulations [12], α may be much greater and the broader region may be covered by loops’ GW. On the other hand, some recent studies [10, 11] show the possibility that α is extremely small, say, α ∼ ( Gµ ) n with n ≳ 1. In such a case, GWs from loops dominate only very high-frequency region and GWs from kinks may be observable at low-frequency region. For example, if α ≲ 10 − $^{9}$, the band of SKA [23] can be used for detection of GWs from kinks, and for Gµ ∼ 10 − $^{7}$, Ω$_{gw}$ 19