2025ApJ...986...49K

Galaxy mergers, each hosting a supermassive black hole (SMBH), are thought to form SMBH binaries. Motivated by recent observations from the East Asian Very Long Baseline Interferometry (VLBI) Network (EAVN) showing periodic behavior in the M87 jet, a precession of about 11 yr and a transverse oscillation of about 0.9 yr, we constrain the mass of a hypothetical secondary black hole orbiting the primary SMBH in M87. To constrain the mass ratio between the primary SMBH (M₁) and the secondary black hole (M₂) defined as q ≡ M₂/M₁ ≤ 1, and the length of the semimajor axis of the binary system (a), we impose the following three constraints: (i) the lower limit of a, below which the SMBH binary is expected to merge; (ii) the strain amplitude of the gravitational-wave background at nanohertz frequencies shown in the NANOGrav 15 yr data set; and (iii) a finite length of the semimajor axis of M₁, which can induce periodic behavior in the jet. By combining these constraints, we obtain the allowed parameter space for q and a. If either of the EAVN-detected periods (T) corresponds to the binary's orbital period, the allowed range of q is 6.9 × 10^–3 ≤ q ≤ 4.2 × 10^–2 for T ≈ 11 yr, and 3.7 × 10^–2 ≤ q ≤ 1 for T ≈ 0.9 yr. VLBI astrometric monitoring of the jet base of M87 is essential to explore the allowed parameter space for q and a.