Probing scalar induced gravitational waves with PTA and LISA: The Importance of third order correction

We revisit the calculation of third order \acp{SIGW} and extend it from a monochromatic primordial power spectrum to a more general log-normal one. We investigate the impact of third order SIGWs on \ac{SNR} of \ac{LISA} and \ac{PTA} observations, and find that third order SIGWs significantly contribute to the total energy density spectrum of \acp{GW} in high-frequency region. For a primordial power spectrum amplitude of $A_{\zeta }={10}^{-2}\sim {10}^{-1}$, the effects of third order SIGWs lead to a $40\mathrm{\%}$ to $400\mathrm{\%}$ increase in the SNR for LISA. Additionally, our PTA data analysis reveals that third order SIGWs diminish both the amplitude $A_{\zeta }$ and the peak frequency $f_{\ast }$ of the primordial power spectrum.