A Cross-Band (X-ray times Optical) Periodicity Search for Supermassive Black Hole Binaries: A Null Result and the First Completeness-Corrected Constraint
Researchers searched large AGN samples for supermassive black hole binaries showing coherent quasiperiodicity across X-ray and optical bands. The search found no co-periodic candidates, establishing a constraint that highlights the need for denser X-ray monitoring to detect such systems in the future.