Nanohertz Gravitational-Wave Signatures from the Supermassive Black Hole Binary Candidate in 3C 454.3

We investigate the orbital dynamics and gravitational-wave (GW) properties of the blazar 3C 454.3 as a candidate supermassive black hole binary (SMBHB). Nearly six decades of multi-frequency radio monitoring reveal a persistent quasi-periodic modulation with a period of 2.3 yr in the observer frame. Interpreting this modulation as a Doppler-boosted signature of orbital motion or jet precession, and applying a relativistic transformation with Lorentz factor γ = 5, we derive an intrinsic orbital period of ≈31 yr in the source frame.

Using Keplerian and relativistic precession models, we estimate component masses of M≈2.4×10^{10}M⊙​ and m≈6.8×10^{9}M⊙​, corresponding to an orbital separation of a≈4.6×10^{17} cm (∼0.15 pc). The system therefore lies firmly in the nanohertz GW regime accessible to pulsar timing arrays (PTAs). The predicted GW frequency is fGW,obs≈1.1×10^{-9} Hz, while the characteristic strain amplitude reaches h0≈(7–9)×10^{-16}, making 3C 454.3 a promising PTA-band source.

The GW-driven inspiral timescale is tGW≈5.7×10^{4} yr. The expected orbital period decay is extremely small, ∣P˙∣≈2×10^{-4} yr yr^{−1}, corresponding to only a few days of cumulative drift over 60 yr. Such a variation is masked by the intrinsic variability of Doppler-boosted jet emission and cannot be detected in current electromagnetic data.

We show that the extreme chirp mass and low GW frequency of 3C 454.3 make it a potentially significant contributor to the anisotropy of the stochastic nanohertz GW background. The source lies far below the sensitivity range of LISA and LGWA, confirming that it is a PTA-only target throughout its inspiral evolution. Overall, 3C 454.3 emerges as one of the most promising high-mass SMBHB candidates for future PTA observations.