2011MNRAS.412...26M

We investigate the spectral properties of an active black hole, member of a massive (10⁷-10⁹ M_☉) 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 Hβ, and compute the flux ratios between the lines of Mg II and C IV ({img}) and those of Mg II and Hβ ({img}). We find that close black hole binaries have {img} up to 1 order of magnitude smaller than single black holes. By contrast {img} 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 ≳ years) would identify genuine sub-pc binary candidates.