Gravitational lensing of gravitational waves: prospects for probing intermediate-mass black holes in galaxy lenses with global minima image

Abstract: This work studies microlensing effects in strongly lensed gravitational wave (GW) signals corresponding to global minima images in galaxy-scale lenses. We find that stellar microlenses alone are unable to introduce noticeable wave effects in the global minima GW signals at strong lensing magnification $(\mu)<50$ with match value between unlensed and lensed GW signals being above ${\sim}99.5\%$ in ${\sim}90\%$ of systems implying that GW signals corresponding to global minima can be treated as reference signal to determine the amount of microlensing in other strongly lensed counterparts. Since the stellar microlenses introduce negligible wave effects in global minima, they can be used to probe the intermediate-mass black hole (IMBH) lenses in the galaxy lens. We show that the presence of an IMBH lens with mass in the range $[50,10^3] {\rm M_\odot}$ such that the global minima lies within five Einstein radius of it, the microlensing effects at $f<10^2$Hz are mainly determined by the IMBH lens for $\mu<50$. Assuming that a typical strong lensing magnification of 3.8 and high enough signal-to-noise ratio (in the range ${\simeq}[10, 30]$) to detect the microlensing effect in GW signals corresponding to global minima, with non-detection of IMBH-led microlensing effects in ${\simeq}15:({\simeq}150)$ lensed GW signals, we can rule out dark matter fraction $>10\%:(>1\%)$ made of IMBH population inside galaxy lenses with mass values $>150 {\rm M_\odot}$ with ${\sim}90\%$ confidence. Although we have specifically used IMBHs as an example, the same analysis applies to any subhalo (or compact objects) with lensing masses (i.e., the total mass inside Einstein radius) satisfying the above criterion.