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Coalescence of super massive black holes (SMBH's) in galaxy mergers ispotentially the dominant contributor to the low frequency gravitational wavebackground (GWB). IIt was proposed by Merritt and Ekers (2002) that X-shapedradio galaxies are signposts of such coalescences, and that their abundancemight be used to predict the magnitude of the gravitational wave background. InRoberts et al. (2015) we present radio images of all 52 X-shaped radio sourcecandidates out of the sample of 100 selected by Cheung (2007) for whicharchival VLA data were available. These images indicate that at most 21% of thecandidates might be genuine X-shaped radio sources that were formed by arestarting of beams in a new direction following a major merger. This suggeststhat fewer than 1.3% of extended radio sources appear to be candidates forgenuine axis reorientations ("spin flips"), much smaller than the 7% suggestedby Leahy and Parma (1992). Thus the associated gravitational wave backgroundmay be substantially smaller than previous estimates. These results can be usedto normalize detailed calculations of the SMBH coalescence rate and the GWB.

The Abundance of X-Shaped Radio Sources: Implications for the Gravitational Wave Background