  Double-peaked broad emission lines in Active Galactic Nuclei (AGNs) may
indicate the existence of a bound supermassive black hole (SMBH) binary where
two distinct broad line regions (BLRs) contribute together to the line profile.
An alternative interpretation is a disk emitter origin for the double-peaked
line profile. Using simple BLR models, we calculate the expected broad line
profile for a SMBH binary at different separations. Under reasonable
assumptions that both BLRs are illuminated by the two active SMBHs and that the
ionizing flux at the BLR location is roughly constant, we confirm the emergence
of double-peaked features and radial velocity drifts of the two peaks due to
the binary orbital motion. However, such a clear double-peaked feature only
arises in a particular stage of the binary evolution when the two BHs are close
enough such that the line-of-sight orbital velocity difference is larger than
the FWHM of the individual broad components, while the two BLRs are still
mostly distinct. Prior to this stage, the velocity splitting due to the orbit
motion of the binary is too small to separate the emission from the two BLRs,
leading to asymmetric broad line profiles in general. When the two BHs are even
closer such that the two BLRs can no longer be distinct, the line profile
becomes more complex and the splitting of the peaks does not correspond to the
orbital motion of the binary. In this regime there are no coherent radial
velocity drifts in the peaks with time. Asymmetric line profiles are probably a
far more common signature of binary SMBHs than are double-peaked profiles. We
discuss the temporal variations of the broad line profile for binary SMBHs and
highlight the different behaviors of reverberation mapping in the binary and
disk emitter cases, which may serve as a feasible tool to disentangle these two
scenarios.
