Numerical relativity simulations predict that coalescence of supermassive black hole ( SMBH ) binaries not only leads to a spin flip but also to a recoiling of the merger remnant SMBHs .
In the literature , X-shaped radio sources are popularly suggested to be candidates for SMBH mergers with spin flip of jet-ejecting SMBHs .
Here we investigate the spectral and spatial observational signatures of the recoiling SMBHs in radio sources undergoing black hole spin flip .
Our results show that SMBHs in most spin-flip radio sources have mass ratio q \gtrsim 0.3 with a minimum possible value q _ { min } \simeq 0.05 .
For major mergers , the remnant SMBHs can get a kick velocity as high as 2100 { km~ { } s ^ { -1 } } in the direction within an angle \lesssim 40 ^ { \circ } relative to the spin axes of remnant SMBHs , implying that recoiling quasars are biased to be with high Doppler-shifted broad emission lines while recoiling radio galaxies are biased to large apparent spatial off-center displacements .
We also calculate the distribution functions of line-of-sight velocity and apparent spatial off-center for spin-flip radio sources with different apparent jet reorientation angles .
Our results show that the larger the apparent jet reorientation angle is , the larger the Doppler-shifting recoiling velocity and apparent spatial off-center displacement will be .
We investigate the effects of recoiling velocity on the dust torus in spin-flip radio sources and suggest that recoiling of SMBHs would lead to “ dust poor ” AGNs .
Finally , we collect a sample of 19 X-shaped radio objects and for each object give the probability of detecting the predicted signatures of recoiling SMBH .