Detecting subparsec supermassive binary black holes: Long-term monitoring perspective

Here we consider the perspective to detect sub-pc supermassive binary black hole (SMBBH) systems using long-term photometric and spectroscopic monitoring campaigns of active galactic nuclei. This work explores the nature of long-term spectral variability caused by the dynamical effects of SMBBH systems. We describe in great detail a model of SMBBH system which considers that both black holes have their accretion disc and additional line emitting region(s). We simulate the H beta spectral band (continuum+broad H beta line) for different mass ratios of components and different total masses of the SMBBH systems (10(6)-10(8)M(circle dot)). We analyse the set of continuum and broad line light curves for several full orbits of SMBBHswith different parameters, to test the possibility to extract the periodicity of the system. We consider different levels of the signal-to-noise ratio, which is added to the simulated spectra. Our analysis showed that the continuum and broad line profiles emitted from an SMBBH system are strongly dependent, not only on the mass ratio of the components but also on the total mass of the system. We found that the mean broad line profile and its rms could indicate the presence of an SMBBH. However, some effects caused by the dynamics of a binary system could be hidden due to a low signal-to-noise ratio. Finally, we can conclude that the long-term AGN monitoring campaigns could be beneficial for the detection of SMBBH candidates.

Automated summary

This study explores the detection of sub-pc supermassive binary black hole systems through long-term photometric and spectroscopic monitoring of active galactic nuclei. By simulating H beta spectral bands for different mass ratios and total masses of the SMBBH systems, the research examines the nature of long-term spectral variability. The analysis finds that continuum and broad line profiles emitted from an SMBBH system are strongly dependent on the mass ratio and total mass of the system, indicating the potential benefits of long-term AGN monitoring campaigns for detecting SMBBH candidates.