Velocity offsets in the broad Balmer lines of quasars and their temporal variations serve as indirect evidence for bound supermassive black hole binaries (SBHBs) at subparsec separations. In this work, we test the SBHB hypothesis for 14 quasars with double-peaked broad emission lines using their long-term (14-41 years) radial velocity curves. We improve on previous work by (a) using elliptical instead of circular orbits for the SBHBs, (b) adopting a statistical model for radial velocity jitter, (c) employing a Markov Chain Monte Carlo method to explore the orbital parameter space efficiently and build posterior distributions of physical parameters and (d) incorporating new observations. We determine empirically that jitter comprises approximately Gaussian distributed fluctuations about the smooth radial velocity curves that are larger than the measurement errors by factors of order a few. We initially treat jitter by enlarging the effective error bars and then verify this approach via a variety of Gaussian process models for it. We find lower mass limits for the hypothesized SBHBs in the range 10 8 -10 11 M . For seven objects the SBHB scenario appears unlikely based on goodnessof-fit tests. For two additional objects the minimum SBHB masses are unreasonably large (> 10 10 M ), strongly disfavoring the SBHB scenario. Using constraints on the orbital inclination angle (which requires some assumptions) makes the minimum masses of four more objects unreasonably large. We also cite physical and observational arguments against the SBHB hypothesis for nine objects. We conclude that the SBHB explanation is not the favoured explanation of double-peaked broad emission lines.

“Changing-look” quasars are a new class of highly variable active galactic nuclei that have changed their spectral type over surprisingly short timescales of just a few years. The origin of this phenomenon is debated, but is likely to reflect some change in the accretion flow. To investigate the disk–corona systems in these objects, we measure optical/UV–X-ray spectral indices ( α OX ) and Eddington ratios ( λ Edd ) of 10 previously discovered changing-look quasars at two or more epochs. By comparing these data with simulated results based on the behavior of X-ray binaries, we find possible similarities in spectral indices below the 1% Eddington ratio. We further investigate the Eddington ratios of changing-look quasars before and after their spectral type changes, and find that changing-look quasars cross the 1% Eddington ratio boundary when their broad emission lines disappear/emerge. This is consistent with the disk-wind model as the origin of broad emission lines.