Search for sub-parsec massive binary black holes through line diagnosis

We investigate the spectral properties of an active black hole, member of a massive (107-109 M-circle dot) sub-parsec black hole binary. We work under the hypothesis that the binary, surrounded by a circumbinary disc, has cleared a gap, and that accretion occurs on to the secondary black hole fed by material closer to the inner edge of the disc. Broad line emission clouds orbit around the active black hole and suffer erosion due to tidal truncation at the Roche Lobe surface, following gap opening and orbital decay. We consider three of the most prominent broad emission lines observed in the spectra of active galactic nuclei (AGNs), i.e. C iv, Mg ii and H beta, and compute the flux ratios between the lines of Mg ii and C iv (<file name="mnr_17888_mu1.gif" type="gif"/>) and those of Mg ii and H beta (<file name="mnr_17888_mu2.gif" type="gif"/>). We find that close black hole binaries have <file name="mnr_17888_mu3.gif" type="gif"/> up to 1 order of magnitude smaller than single black holes. By contrast <file name="mnr_17888_mu4.gif" type="gif"/> may be significantly reduced only at the shortest separations. Peculiarly low values of line flux ratios together with large velocity offsets between the broad and narrow emission lines and/or periodic variability in the continuum (on time-scales greater than or similar to years) would identify genuine sub-pc binary candidates.