Search of sub-parsec massive binary black holes through line diagnosis
  II
  Massive black hole binaries at sub-parsec separations may display in their
spectra anomalously small flux ratios between the MgII and CIV broad emission
lines, i.e. F_MgII/F_CIV <~ 0.1, due to the erosion of the broad line region
around the active, secondary black hole, by the tidal field of the primary. In
Paper I by Montuori et al. (2011), we focussed on broad lines emitted by gas
bound to the lighter accreting member of a binary when the binary is at the
center of a hollow density region (the gap) inside a circum-binary disc. The
main aim of this new study is at exploring the potential contribution to the
broad line emission by the circum-binary disc and by gaseous streams flowing
toward the black hole through the gap. We carry out a post-process analysis of
data extracted from a SPH simulation of a circum-binary disc around a black
hole binary. Our main result is that the MgII to CIV flux ratio can be reduced
to ~ 0.1 within an interval of sub-pc binary separations of the order of a ~
(0.01-0.2)(f_Edd/0.1)^(1/2) pc corresponding to orbital periods of ~ (20-200)
(f_Edd/0.1)^(3/4) years for a secondary BH mass in the range M_2 ~ 10^7-10^9
M_sun and a binary mass ratio of 0.3. At even closer separations this ratio
returns to increase to values that are indistinguishable from the case of a
single AGN (typically F_MgII/F_CIV ~ 0.3-0.4) because of the contribution to
the MgII line from gas in the circum-binary disc.
