Broad-line region configuration of the supermassive binary black hole
  candidate PG1302-102 in the relativistic Doppler boosting scenario
  PG1302-102 is thought to be a supermassive binary black hole (BBH) system
according to the periodical variations of its optical and UV photometry, which
may be interpreted as being due to the relativistic Doppler boosting of the
emission mainly from the disk around the secondary black hole (BH) modulated by
its orbital motion. In this paper, we investigate several broad emission lines
of PG1302-102 using archived UV spectra obtained by IUE, GALEX, and Hubble, to
reveal the broad-line region (BLR) emission properties of this BBH system under
the Doppler boosting scenario. We find that the broad lines Ly$\alpha$, NV,
CIV, and CIII] all show Gaussian profiles, and none of these lines exhibits
obvious periodical variation. Adopting a simple model for the BLR, we perform
Markov chain Monte Carlo fittings to these broad lines, and find that the BLR
must be viewed at an orientation angle of $\sim33^{\circ}$, close to face-on.
If the Doppler boosting interpretation is correct, then the BLR is misaligned
with the BBH orbital plane by an angle of $\sim51^\circ$, which suggests that
the Doppler boosted continuum variation has little effect on the broad-line
emission and thus does not lead to periodical line variation. We further
discuss the possible implications for such a BLR configuration with respect to
the BBH orbital plane.
