The upcoming LISA gravitational wave detector is expected to observe a phenomenon known as extreme mass ratio inspirals (EMRIs), in which small black holes spiral into supermassive ones at the centre of galaxies. These interactions produce gravitational waves, which LISA will detect in a different frequency range to ground-based detectors such as LIGO. Scientists expect LISA to observe a 'rain' of these events. There could be tens or even hundreds of events during the mission. In scenarios where two supermassive black holes interact in galactic mergers, thousands of EMRI events could occur, with a few hundred detectable by LISA. This expected flood of observations could shed light on black hole masses and galaxy properties, possibly confirming the presence of multiple supermassive black holes in certain galaxies and correlating these events with post-starburst galaxy properties. Identifying and studying these EMRIs could provide important insights into the dynamics and evolution of black holes within galaxies.
https://skyandtelescope.org/astronomy-blogs/black-hole-files/black-hole-rain/