Strongly Lensed Supermassive Black Hole Binaries as Nanohertz Gravitational-Wave Sources

Abstract

Supermassive black hole binary systems (SMBHBs) should be the most powerful sources of gravitational waves in the Universe. Once Pulsar Timing Arrays (PTAs) detect the stochastic gravitational-wave background from their cosmic merger history, searching for individually resolvable binaries will take on new importance. Since these individual supermassive black hole binary systems are expected to be rare, here we explore how strong gravitational lensing can act as a tool for increasing their detection prospects by magnifying fainter sources and bringing them into view. We investigate gravitational wave signals from SMBHBs that might be detectable with current and future PTAs under the assumption that quasars serve as bright beacons that signal a recent merger. Using the black hole mass function derived from quasars and assuming fortuitous alignment yielding a high magnification factor of $\mu = 30$, we can expect to detect of the order of 1 strongly lensed binary system out to $z \approx 1.25$. Encouragingly, though, to within $z = 2$, strong lensing adds $\sim$9$-$26 more detectable binaries for PTAs. Finally, we investigate the possibility of observing both time-delayed electromagnetic signals and gravitational wave signals from these strongly lensed binary systems - that will provide us with unprecedented multi-messenger insights into their orbital evolution.