Can we disentangle between the emission of an accretion disc around a single black hole and a circumbinary disc?

ABSTRACT The detection of gravitational waves from binary black holes (BBHs) started the hunt for their pre-merger electromagnetic emission. In that respect, numerical simulations have been looking for the ‘smoking gun’ signal that could help identify pre-merger systems. Here, we study if any of the expected features of circumbinary discs, such as the periodic modulation from the orbiting ‘lump’, could be used to identify pre-merger BBHs or if they could be easily confused with other systems. Indeed, while the timing feature associated with the ‘lump’ seems to be present for a large part of the parameter space defined by the binary separation and mass ratio in circular binaries, it was recently proposed to form thanks to an instability occurring naturally at the edge of accretion discs around single black holes (SBH). In order to check if features of a circumbinary disc could be reproduced by a SBH system, we search for at least one SBH fit able to replicate the given synthetic observations of a circumbinary disc. We found that many of the features from a circumbinary disc can be reproduced by an SBH system with different masses, distances, or inner disc positions. Interestingly, while we can always find an SBH model providing a good enough fit to the data, the presence of two variabilities, associated with the lump and the binary, or binary-lump beat, period, is a necessary condition for a wide range of BBH system parameters and should be used as a test to disqualify some BBH candidates.