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

TL;DR

This paper explores spectral signatures of sub-parsec massive black hole binaries, proposing line flux ratios and variability as indicators to identify such systems in active galactic nuclei.

Contribution

It introduces a model linking specific emission line flux ratios and variability patterns to the presence of close black hole binaries, aiding their detection.

Findings

FMgII/FCIV ratios are significantly lower in binaries than in single black holes.

Shorter separations lead to notable reductions in FMgII/FHβ ratios.

Line flux ratios combined with velocity offsets and variability can identify binary candidates.

Abstract

We investigate on the spectral properties of an active black hole, member of a massive (10^7 - 10^9 Msun) sub-parsec black hole binary. We work under the hypothesis that the binary, surrounded by a circum-binary disc, has cleared a gap, and that accretion occurs onto 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 AGNs, i.e. CIV, MgII and H{\beta}, and compute the flux ratios between the lines of MgII and CIV (FMgII/FCIV) and those of MgII and H{\beta} (FMgII/FH{\beta}). We find that close black hole binaries have FMgII/FCIV up to one order of magnitude smaller than single black holes. By contrast FMgII/FH{\beta} 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 timescales >= years) would identify genuine sub-pc binary candidates.