Rapid advancement in the observation of cosmic strings has been made in recent years placing increasingly stringent constraints on their properties , with G \mu \lesssim 10 ^ { -11 } from Pulsar Timing Array ( PTA ) observations .
Cosmic string loops with low string tension clump in the halo of the Galaxy due to the combination of slow loop decay and low gravitational recoil , resulting in great enhancement to loop abundance in the Galaxy .
With an average separation of down to just a fraction of a kpc , and the total power of gravitational wave ( GW ) emission dominated by harmonic modes spanning a wide angular scale , resolved loops located in proximity to the solar system are powerful , persistent , and highly monochromatic sources of GW with a harmonic signature not replicated by any other sources , making them prime targets for direct detection by the upcoming Laser Interferometer Space Antenna ( LISA ) , whose frequency range is well-matched for this task .
Unlike detection of bursts where the detection rate scales with loop abundance , the detection rate for harmonic signal is the result of a complex interplay between the strength of GW emission by a loop , loop abundance , and other sources of noise , and is most suitably investigated through numerical simulations .
We develop a robust and flexible framework for simulating loops in the Galaxy for predicting direct detection of harmonic signal from resolved loops by LISA .
Our simulation reveals that the most accessible region in the parameter space for direct detection consists of large loops \alpha = 0.1 with low string tension 10 ^ { -21 } \lesssim G \mu \lesssim 10 ^ { -19 } .
Direct detection of field theory cosmic strings is unlikely , with the detection probability p _ { \mathrm { det } } \lesssim 2 \% for a 1-year mission under the most favorable conditions .
An extension of our results suggests that direct detection of cosmic superstrings with a low intercommutation probability is very promising , even unavoidable with optimal parameters .
Searching for harmonic GW signal from resolved loops through LISA observations will potentially place physical constraints on string theory .