Looking for Supermassive Black Hole Binaries with a Flash of Starlight
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Supermassive black hole binaries can be difficult to detect in many galaxies, but a new approach could find them by looking for the regular flashes of starlight caused by the gravitational lensing of these black holes.
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Looking for Supermassive Black Hole Binaries with a Flash of Starlight By Brian Koberlein - March 11, 2026 03:11 PM UTC | Black Holes Most galaxies have a supermassive black hole at their center, but some galaxies have two. These supermassive binaries form when two galaxies collide and merge. We can detect some of these binaries, such as by observing the periodic changes of a quasar or by observing the binary directly, such as in the case of NGC 7727. But most supermassive binaries remain hidden. They are too far away to be observed directly or too inactive to be observed by jets. And while gravitational wave observatories can detect the mergers of stellar-mass black holes, we can't yet detect the mergers of supermassive black holes. But a new study shows how we might detect some of them. The idea is based on gravitational lensing, where the mass of an object deflects a path of light similar to the way the glass lens of a telescope focuses starlight. Gravitational lensing is most commonly seen when a distant quasar is lensed by a foreground galaxy. A similar effect known as microlensing occurs when a mass passes in front of a more distant star. In this case, the study looks at what would happen as the supermassive black holes orbit each other against the background of stars within their galaxy. The black holes always lens nearby starlight, but when the two black holes are positioned just so, they can act as a particularly strong lens. This could cause a background star to appear much brighter than usual. Since this alignment occurs every orbit or half orbit, the brightness would occur periodically. How binary black holes can gravitationally lens a background star. Credit: Wang, et al. It's not quite this simple, since there could be more than one star with just the right alignment, and the black holes and stars are all moving, so alignments would change over time. But the authors show that quasiperiodic flashes could still occur. Based on simulations, the authors pr...
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