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The bright quasar PG1302-102 has been identified as a candidate supermassiveblack hole binary from its near-sinusoidal optical variability. While thesignificance of its optical periodicity has been debated due to the stochasticvariability of quasars, its multi-wavelength variability in the ultraviolet(UV) and optical bands is consistent with relativistic Doppler boost caused bythe orbital motion in a binary. However, this conclusion was based previouslyon sparse UV data which were not taken simultaneously with the optical data.Here we report simultaneous follow-up observations of PG1302-102 with theUltraviolet Optical Telescope on the Neil Gehrels Swift Observatory in sixoptical + UV bands. The additional nine Swift observations produce light curvesroughly consistent with the trend under the Doppler boost hypothesis, whichpredicts that UV variability should track the optical, but with a ~2.2 timeshigher amplitude. We perform a statistical analysis to quantitatively test thishypothesis. We find that the data are consistent with the Doppler boosthypothesis when we compare the the amplitudes in optical B-band and UV lightcurves. However, the ratio of UV to V-band variability is larger than expectedand is consistent with the Doppler model, only if either the UV/opticalspectral slopes vary, the stochastic variability makes a large contribution inthe UV, or the sparse new optical data underestimate the true opticalvariability. We have evidence for the latter from comparison with the opticallight curve from ASAS-SN. Additionally, the simultaneous analysis of all fourbands strongly disfavors the Doppler boost model whenever Swift V-band isinvolved. Additional, simultaneous optical + UV observations tracing outanother cycle of the 5.2-year proposed periodicity should lead to a definitiveconclusion.

Testing the relativistic Doppler boost hypothesis for the binary candidate quasar PG1302-102 with multi-band Swift data