Testing the relativistic Doppler boost hypothesis for supermassive
  binary black holes candidates via broad emission line profiles
  Optical periodicity QSOs found by transient surveys are suggested to be
sub-parsec supermassive binary black holes (BBHs). An intriguing interpretation
for the periodicity of some of those QSOs is that the continuum is radiated
from the accretion disk associated with the BBH secondary component and
modulated by the periodical rotation of the secondary via Doppler-boost effect.
Close to edge-on orbital orientation can lead to more significant Doppler-boost
effect and thus are preferred for these systems, which is distinct from those
normal type-1 QSOs with more or less face-on orientations. Therefore, the
profiles of broad lines emitted from these Doppler-modulated systems may be
significantly different from other systems that are not Doppler-modulated. We
investigate the properties of the broad emission lines of optical-periodicity
QSOs, including both a sample of QSOs that can be interpreted by the
Doppler-modulated effects and a sample that cannot. We find that there is no
obvious difference in the profiles and other properties of various (stacked)
broad emission lines of these two samples, though a simple broad line region
model would suggest significant differences. Our finding raises a challenge to
the Doppler boost hypothesis for some of those BBHs candidates with optical
periodicity.
