Black Hole Mergers from Globular Clusters Observable by LISA and LIGO: Results from post-Newtonian Binary-Single Scatterings

We study the gravitational wave (GW) frequency and chirp mass distribution of binary black hole (BBH) mergers assembled through three-body interactions in globular clusters (GCs), when GW emission at the 2.5 post-Newtonian (PN) level is included in the $N$-body equation-of-motion (EOM). From performing $\sim 2.5\times10^{6}$ PN binary-single interactions based on GC data from the `MOCCA-Survey Database I' project, and by the use of analytical methods, we find that $5-10\%$ of all the three-body assembled GC BBH mergers have a GW frequency at formation that is $\gtrsim 10^{-1}$ Hz, implying they enter the LIGO band without having drifted through the LISA band first. If PN terms are not included in the EOM, one finds instead that all BBH mergers drifts through both LISA and LIGO. As the fraction of BBH mergers that only show up in LIGO is expected to be $\sim 0\%$ for standard field binary BBH mergers, future joint measurements with LISA and LIGO can be used to gain insight into the formation of BBH mergers.