Spectral Energy Distributions of Candidate Periodically-Variable
  Quasars: Testing the Binary Black Hole Hypothesis
  Periodic quasars are candidates for binary supermassive black holes (BSBHs)
efficiently emitting low frequency gravitational waves. Recently, $\sim$150
candidates were identified from optical synoptic surveys. However, they may be
false positives caused by stochastic quasar variability given the few cycles
covered (typically 1.5). To independently test the binary hypothesis, we search
for evidence of truncated or gapped circumbinary accretion discs (CBDs) in
their spectral energy distributions (SEDs). Our work is motivated by CBD
simulations that predict flux deficits as cutoffs from central cavities opened
by secondaries or notches from minidiscs around both BHs. We find that
candidate periodic quasars show SEDs similar to those of control quasars
matched in redshift and luminosity. While seven of 138 candidates show a blue
cutoff in the IR-optical-UV SED, six of which may represent CBDs with central
cavities, the red SED fraction is similar to that in control quasars,
suggesting no correlation between periodicity and SED anomaly. Alternatively,
dust reddening may cause red SEDs. The fraction of extremely radio-loud
quasars, e.g., blazars (with $R>100$), is tentatively higher than that in
control quasars (at 2.5$\sigma$). Our results suggest that, assuming most
periodic candidates are robust, IR-optical-UV SEDs of CBDs are similar to those
of accretion discs of single BHs, if the periodicity is driven by BSBHs; the
higher blazar fraction may signal precessing radio jets. Alternatively, most
current candidate periodic quasars identified from few-cycle light curves may
be false positives. Their tentatively higher blazar fraction and lower
Eddington ratios may both be caused by selection biases.
