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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 sub-parsec 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 $10^8$--$10^{11}\;M_{\odot}$. For seven objects the SBHB scenario appears unlikely based on goodness-of-fit tests. For two additional objects the minimum SBHB masses are unreasonably large ($>10^{10}\;M_{\odot}$), 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., Comment: 27 pages, accepted by MNRAS

We have been spectroscopically monitoring 88 quasars selected to have broad H$\beta$ emission lines offset from their systemic redshift by thousands of km s$^{-1}$. By analogy with single-lined spectroscopic binary stars, we consider these quasars to be candidates for hosting supermassive black hole binaries (SBHBs). In this work we present new radial velocity measurements, typically 3-4 per object over a time period of up to 12 years in the observer's frame. In 29/88 of the SBHB candidates no variability of the shape of the broad H$\beta$ profile is observed, which allows us to make reliable measurements of radial velocity changes. Among these, we identify three objects that have displayed systematic and monotonic velocity changes by several hundred km s$^{-1}$ and are prime targets for further monitoring. Because the periods of the hypothetical binaries are expected to be long, we cannot hope to observe many orbital cycles during our lifetimes. Instead, we seek to evaluate the credentials of the SBHB candidates by attempting to rule out the SBHB hypothesis. In this spirit, we present a method for placing a lower limit on the period, and thus the mass, of the SBHBs under the assumption that the velocity changes we observe are due to orbital motion. Given the duration of our monitoring campaign and the uncertainties in the radial velocities, we were able to place a lower limit on the total mass in the range $4.7\times10^4-3.8\times10^8$ $M_{\scriptscriptstyle \odot}$, which does not yet allow us to rule out the SBHB hypothesis for any candidates., Comment: Accepted for publication in MNRAS, full version of Table 1 will be available in MNRAS online version of the paper

We present new spectroscopic observations that are part of our continuing monitoring campaign of 88 quasars at z<0.7 whose broad H$\beta$ lines are offset from their systemic redshifts by a few thousand km/s. These quasars have been considered candidates for hosting supermassive black hole binaries (SBHBs) by analogy with single-lined spectroscopic binary stars. We present the data and describe our improved analysis techniques, which include an extensive evaluation of uncertainties. We also present a variety of measurements from the spectra that are of general interest and will be useful in later stages of our analysis. Additionally, we take this opportunity to study the variability of the optical continuum and integrated flux of the broad H$\beta$ line. We compare the variability properties of the SBHB candidates to those of a sample of typical quasars with similar redshifts and luminosities observed multiple times during the Sloan Digital Sky Survey. We find that the variability properties of the two samples are similar (variability amplitudes of 10-30% on time scales of approximately 1-7 years) and that their structure functions can be described by a common model with parameters characteristic of typical quasars. These results suggest that the broad-line regions of SBHB candidates have a similar extent as those of typical quasars. We discuss the implications of this result for the SBHB scenario and ensuing constraints on the orbital parameters., Comment: 19 pages in ApJ format. Accepted for publication in the Astrophysical Journal Supplements