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We present the study of multi-messenger signatures of massive black hole(MBH) binaries residing in the centres of galaxy merger remnants. Inparticular, we first focus on the gravitational wave background (GWB) producedby an ensemble of MBH binary inspirals in the frequency range probed by thePulsar Timing Array (PTA) experiments. The improved estimates of thecharacteristic strain were obtained with the inclusion of environmental effectson the MBH binary orbital decay within the galaxy merger remnants, added inpost-processing to the semi-analytic model of galaxy formation and evolutionSHARK. Secondly, we explore two, intriguing in terms of the MBH binaryevolution studies, hypotheses aiming to explain the origins of X-shape radiogalaxies - a peculiar type of objects with double lobe structures, constitutingapproximately 6 - 10% of known radio loud galaxies. The two consideredscenarios involve either an abrupt change in the jet direction after a MBHmerger (a spin-flip) or an unresolved close binary, where each of the twocomponents produces a jet. We find that the estimated GWB amplitude at thereference frequency $f_0=1 \,{\rm yr}^{-1}$ is in the range of $A_{\rm{yr^{-1}}} = 1.20\cdot10^{-15} - 1.46\cdot10^{-15}$, which is 50% lower than thestrain of the signal detected by the PTA experiments. We also show that thespin-flip scenario considered in gas-poor mergers reproduces the observedproperties of X-shape radio galaxies well in terms of flip angle, redshift andluminosity distributions.

Massive black hole binaries as sources of low-frequency gravitational waves and X-shape radio galaxies