SIMBAD references

We present a new observational test to identify massive black hole binaries in large multi-epoch spectroscopical catalogues and to confirm already proposed binary candidates. The test is tailored for binaries with large enough separations to allow each black hole to retain its own broad line region (BLR). Within this limit, the fast variability of active galactic nuclei (AGN) typically observed over months cannot be associated to the much longer binary period and is assumed (as for the case of single black holes) to be the consequence of the evolution of the innermost regions of the two accretion discs. A simple analysis of the cross-correlation between different parts of individual broad emission lines can therefore be used to identify the presence of two massive black holes whose continua vary independently of each other. Our analysis indicates that, to be less affected by the noise in the spectra, the broad lines should be divided into two parts of almost equal flux. This ensures that, in the single massive black hole scenario, the cross-correlation will always be strong. With monitoring campaigns similar to those performed for reverberation mapping studies, inversely, a binary can show any value of the cross-correlation and can therefore be distinguished from a standard AGN. This new test can be performed over timescales that are orders of magnitude shorter than the alternative tests already discussed in the literature, and can be a powerful complement to the massive black hole binary search strategies already in place.