It has been confirmed that there are many galaxy mergers where a collision occurs on kpc scale. Two colliding galaxies should evolve into sub-pc phase, where two supermassive black holes (SMBHs) build a super-massive binary black hole (SMBBH) system. This phase is one before coalescence of two SMBHs that is expected to be a source of gravitational waves (GWs). Therefore, the sub-pc SMBBH detection and location on the sky are very important in order to support existing and future GW mission (telescopes). As e.g. future superior radio facilities such as Five hundred-meter Aperture Spherical Telescope (FAST) and Square Kilometre Array (SKA) are going to improve the sensitivity of Pulsar Timing Array (PTA) project for GW detection (see e.g Lam 2018; Nguyen et al. 2020).
However, detection of the sub-pc SMBBHs is a very hard task, first of all, it is not possible to resolve the SMBBH components with current telescopes (as it possible in the case of kpc mergers) . Therefore, some other methods should be used to detect the sub-pc SMBBHs, as e.g. long term spectral observations of promising candidates for SMBBHs (Popovic et al. 2021).
Here we consider the perspective to detect sub-pc SMBBH systems using long-term photometric and spectroscopic monitoring campaigns of active galactic nuclei - AGNs (see Popovic 2012, Popovic et al. 2021). We present a theoretical model, which is including empirical observational constraints and simulated the light curves of SMBBHs. The simulated light curves have been compared with observations. We found that the mean broad line profile and its rms could indicate the presence of an SMBBH and these effects could be observed in line profiles of some AGNs. However, some effects caused by the dynamics of a binary system (as e.g. periodicity in the light curves) could be hidden due to a low signal-to-noise ratio and/or red noise caused by intrinsic variability in a component of SMBBH systems.
References
[1] Lam, M. T., 2018, ApJ, 868, 33
[2] Nguyen, K., Bogdanovic, T., Runnoe, J.C., Taylor, S.R., Sesana, A., Eracleous, M., Sigurdsson, S., 2020, ApJ, 900L, 42
[3] Popović, L. Č, 2012, NewAR, 56, 74
[4] Popović, L. Č, Simić, S., Kovačević, A., Ilić, D. 2021, MNRAS, 505, 5192