2025A&A...703A..23B

Context. A dual active galactic nucleus candidate with a separation of only ≈500 pc was recently found in NGC 7727. According to the hierarchical merging scenario, such objects would be expected to merge on a timescale of a few hundred million years (Myr). However, estimating the accurate merging timescales for the two nuclei is still a complex challenge.
Aims. Using our numerical N-body code, we can trace the full evolution of central black holes during all phases: dynamical friction of unbound black holes, binary black hole formation, hardening of the system due to two-body scattering, and emission of gravitational waves leading to the final merger.
Methods. According to the extracted observational data, the numerical model has three main components: the bulge contains two dense stellar nuclei, each of which hosts a black hole. The observed system is in an advanced stage of merging, where the most massive black hole in the center of the galaxy has a mass of 1.54 × 10⁸ M_⊙ and the least massive black hole in the offset second stripped nucleus has a mass of 6.33 × 10⁶ M_⊙. Using a direct N-body φ-GPU code, we followed the dynamical evolution of the system up to a final separation of four Schwarzschild radii. The black holes were added as special relativistic particles and their equation of motion contains a full post-Newtonian approximation up to the 2.5 term.
Results. Initially, the black holes are not gravitationally bound and, thus, the system spends more than 60 Myr in the phase of dynamical friction while tightening the orbit. Then, a bound binary forms quickly with a relatively high eccentricity of e ≈ 0.98. The two-body scattering phase takes place from ≈60 Myr up to ≈120 Myr. In the last ≈10 Myr, the black hole's separation is seen to be rapidly shrinking due to the gravitational wave emission. Starting from the physical separation observed today, the total merging time in our model is 130 ± 10 Myr, which is less than half the value of the previous estimates.
Conclusions. Our study points to the possibility of the binary black holes of NGC 7727 merging on a relatively short timescale. These results have implications for the statistics of strong sources of gravitational waves at low frequencies, namely, systems engaged in an advanced state of merging (similarly to the case of NGC 7727) are expected to be prime sources for the LISA mission to observe.