Krause, Martin G. H.Shabala, Stanislav SHardcastle, MBicknell, GeoffreyBohringer, HansChon, GayoungNawaz, Mohammad A.Sarzi, MarcWagner, Alexander2022-05-052022-05-050035-8711http://hdl.handle.net/1885/264560Supermassive black hole binaries may be detectable by an upcoming suite of gravitational wave experiments. Their binary nature can also be revealed by radio jets via a short-period precession driven by the orbital motion as well as the geodetic precession at typically longer periods. We have investigated Karl G. Jansky Very Large Array and Multi-Element Radio Linked Interferometer Network (MERLIN) radio maps of powerful jet sources for morphological evidence of geodetic precession. For perhaps the best-studied source, Cygnus A, we find strong evidence for geodetic precession. Projection effects can enhance precession features, for which we find indications in strongly projected sources. For a complete sample of 33 3CR radio sources, we find strong evidence for jet precession in 24 cases (73 per cent). The morphology of the radio maps suggests that the precession periods are of the order of 106-107 yr. We consider different explanations for the morphological features and conclude that geodetic precession is the best explanation. The frequently observed gradual jet angle changes in samples of powerful blazars can be explained by orbital motion. Both observations can be explained simultaneously by postulating that a high fraction of powerful radio sources have subparsec supermassive black hole binaries. We consider complementary evidence and discuss if any jetted supermassive black hole with some indication of precession could be detected as individual gravitational wave source in the near future. This appears unlikely, with the possible exception of M87.MJH acknowledges support from the UK Science and Technology Facilities Council [ST/M001008/1]. HB and GC acknowledge support from the DFG Transregio Programme TR33 and the Munich Excellence Cluster ‘Structure and Evolution of the Universe’. The contribution of GVB, MAN, and AYW to this research was supported by the Australian Research Council Discovery Project, The Key Role of Black Holes in Galaxy Evolution, DP140103341. MAN acknowledges support from a grant of the Brazilian Agency FAPESP (2015/25126-2). The work of AYW has been supported in part by ERC Project No. 267117 (DARK) hosted by Universite´ Pierre et Marie Curie (UPMC) – Paris 6, PI J. Silk. This work benefited from support by the International Space Science Institute, Bern, Switzerland, through its International Team programme ref. no. 393 The Evolution of Rich Stellar Populations & BH Binaries (2017–18)application/pdfen-AU© 2018 The authorsblack hole physicsgravitational wavesgalaxies: jetsradio continuum: galaxies.201910.1093/mnras/sty25582020-12-27