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

Abstract

Massive black hole (BH) binaries at sub‐parsec separations may display in their spectra anomalously small flux ratios between the Mg ii and C iv broad emission lines, i.e. , due to the erosion of the broad line region around the active, secondary BH, by the tidal field of the primary. In Paper I, we focused on broad lines emitted by gas bound to the lighter accreting member of a binary when the binary is at the centre of a hollow density region (the gap) inside a circum‐binary disc. The main aim of this new study is to explore the potential contribution to the broad line emission by the circum‐binary disc and by gaseous streams flowing towards the BH through the gap. We carry out a post‐process analysis of data extracted from a smoothed particle hydrodynamic simulation of a circum‐binary disc around a BH binary. Our main result is that the Mg ii to C iv flux ratio can be reduced to ∼0.1 within an interval of sub‐parsec binary separations of the order of a ∼ (0.01–0.2)(fEdd/0.1)1/2 pc corresponding to orbital periods of ∼(20–200)(fEdd/0.1)3/4 yr for a secondary BH mass in the range M2 ∼ 107–109 M⊙ and a binary mass ratio of 0.3. At closer separations this ratio increases to values that are indistinguishable from the case of a single active galactic nucleus (typically ) because of the contribution to the Mg ii line from gas in the circum‐binary disc.