  The bright quasar PG1302-102 has been identified as a candidate supermassive
black hole binary from its near-sinusoidal optical variability. While the
significance of its optical periodicity has been debated due to the stochastic
variability of quasars, its multi-wavelength variability in the ultraviolet
(UV) and optical bands is consistent with relativistic Doppler boost caused by
the orbital motion in a binary. However, this conclusion was based previously
on sparse UV data which were not taken simultaneously with the optical data.
Here we report simultaneous follow-up observations of PG1302-102 with the
Ultraviolet Optical Telescope on the Neil Gehrels Swift Observatory in six
optical + UV bands. The additional nine Swift observations produce light curves
roughly consistent with the trend under the Doppler boost hypothesis, which
predicts that UV variability should track the optical, but with a ~2.2 times
higher amplitude. We perform a statistical analysis to quantitatively test this
hypothesis. We find that the data are consistent with the Doppler boost
hypothesis when we compare the the amplitudes in optical B-band and UV light
curves. However, the ratio of UV to V-band variability is larger than expected
and is consistent with the Doppler model, only if either the UV/optical
spectral slopes vary, the stochastic variability makes a large contribution in
the UV, or the sparse new optical data underestimate the true optical
variability. We have evidence for the latter from comparison with the optical
light curve from ASAS-SN. Additionally, the simultaneous analysis of all four
bands strongly disfavors the Doppler boost model whenever Swift V-band is
involved. Additional, simultaneous optical + UV observations tracing out
another cycle of the 5.2-year proposed periodicity should lead to a definitive
conclusion.
